Drivetrain noise
 

Happen to have this odd drivetrain noise. It is louder than it sounds on the video.

Link to the video: https://www.youtube.com/watch?v=9plCftB58Jg

When the RPMs drop (ex. from 3rd to 4th gear) you can hear the loud humming/grinding noise. It can be duplicated in all gears. Above approx. 1000 rpm it fades away.

This is the 2nd SM420, the previous one had the same sound. I'm suspecting the clutch or the drivetrain, not the gearbox. It sounds like it comes from the transmission.

I'm thinking rattling clutch springs or something like that. Haven't had the clutch apart yet.

I've tried to pinpoint the noise by looking around on youtube but cannot find anything remotely close to it.

FYI: SM420 is in good order. The drivetrain has new u joints and driveshaft support bearing.

Appreciate any input. Thanks.

Re: Drivetrain noise
 

Humming/grinding noise does not do it for me as a description. It's very hard to tell which of the noises I hear is the one you are asking about. I hear a low pitch rumble and a higher pitch "whine". First sounds like exhaust. Second sounds like normal gear mesh noise. Sounds like many old trucks I've had. But noise from a video/cell phone is very difficult.

All I can say is experiment with the clutch pedal. Push clutch pedal in when the noise starts and see if it goes away. What noises are there when down shifting. Do some coasting with clutch pedal in to see what noises you hear at different coast speeds. Try to run on different era paved roads. Do you have cruise/car style tread tires? or snow/mud..... or radical tread design? Bias, radial....

Do you feel any vibration frequency that matches the sound frequency/pitch/tone?
What viscosity lube in trans? Synthetic lube. "GL" number?

What outside temp do you have now?

Re: Drivetrain noise
 

To clarify, the noise appears at 11 seconds into the video and lasts until 17 seconds into the video. It sounds like something is grinding, or a low pitch rumble as you said Ken.

It could be exhaust but I've switched mufflers and it is still there.

This happens when the RPMs drop (after shifting). The usual SM420 gear whine is apparent in the video but not what I'm talking about. In 4th gear there should be barely any tranny noise because it is a 1:1 and the gears are not subjected to a load.

What noises are there when downshifting? No noise like this, just the gear whine when shifting to lower gears.

It happens in all types of roads. The tires are Toyo open country 235/85 R16 radials, new tires. Not a off road type by any means, no radical threads.

Do you feel any vibration frequency that matches the sound frequency/pitch/tone? No, nothing is felt other than hearing that noise.

What viscosity lube in trans? Synthetic lube. "GL" number? straight 90W GL4 mineral lube.

What outside temp do you have now? About 43 degrees Fahrenheit.

Will do some testing with the clutch pedal.

Re: Drivetrain noise
 

I guess I hear it......at 75 with hearing aids. Anyway would be nice to see what happens at 11 sec noise if clutch pedal was pushed in. What do you know about the throw out bearing? But they make noise when clutch pedal depressed.

So the noise is in 4th gear, after clutch released? and slight acceleration?

It may be one of those things that has to get worse to analyze. Watch the gauges. Check for smells and leaking fluids. Very hard to guess.

Re: Drivetrain noise
 

Throw out bearing is in good condition, checked that when I swapped the tranny. No noises when the clutch is depressed.

So the noise is in 4th gear, after clutch released? and slight acceleration?

Yes, 4th gear, low rpms, after the clutch is released and power is transferred through the drivetrain. Up a certain RPM and then it disappears.

I forgot to mention earlier that the slip yoke is pretty sloppy. Maybe that could come into play as well...

Re: Drivetrain noise
 

Should be using this:
https://www.summitracing.com/parts/p...YaAn6zEALw_wcB

GL4 has too much sulfur in it and will eat up the brass synchro. GL5 is worst.

Re: Drivetrain noise
 

Do you mean the splines of the slip yoke or the Ujoint of the slip yoke?

Put your hand on the trans shift tower, where stick come out and see if you feel the noise. Stick shake.
You are an engineer, you know about natural frequency. This can be similar. You mention it starts at a certain RPM and stops and another. This makes me think of amplitude, resonance. High frequency "rattle" sounding like gear or bearing problem, but is not.

The 2, 3 and 4th gears are helical and don't make much noise. But they go make some noise. Noise at 1:1 is said to either be normal in SM420 or bearings. Additives may quiet it. Lucas is said to be best.
Old trucks with all metal cabs and no sound deadener, have always made noise. They are a huge transmitter.
Make sure ujoints are phased.

Re: Drivetrain noise
 

I've always been told that GL4 is used for gearboxes with brass synchro's and GL5 is harsh on said parts. I have GL5 in the rear axle. I'll have to change the oil anyway, will look into it.

Re: Drivetrain noise
 

Slip yoke splines. U joints are new.

U joints are phased now, I am going to turn the splines 180 degrees and see if that helps any. Then they are still phased but mirrored.

Thanks for the help, Ken. I will go out and experiment some, I'll report back my findings.

Re: Drivetrain noise
 

Do you use this oil in your SM420? How does it shift?

Re: Drivetrain noise
 

It works very well. I have almost 8,000 miles on the transmission (that how long I own it) with that oil. Up/down shift like it should and no unusual noise.

Re: Drivetrain noise
 

Update: weather was pretty bad but has cleared up now it seems. Been driving the truck and tried to fiddle with the clutch pedal, but have not managed to 'remove' that noise. When I depress the clutch pedal slightly when accelerating it starts to slip (makes sense, of course) but that does not give me anything.

Will keep trying and if I find out I'll be sure to post it.

Re: Drivetrain noise
 

Update:

I took some videos regarding the play. Both are with the truck in gear.
The slip yoke has some slop, but not excessive I think. I switched the drive shaft on the yoke 180 degrees but did this not have any effect on the noise.

Slip yoke link: https://youtu.be/7CverabZ7Kk

The rear end has a lot more play though. But it is a total play, as the floating axle splines probably have some play as well, as do the gears inside diff. The pinion shaft is really tight, with no up and down slop or axial play.

Link: https://youtu.be/nKUeE7NV5gc

Any thoughts?

Re: Drivetrain noise
 

In my opinion the slip joint is worn out.

The ring and pinion will need further investigation (that's too much play). With a clean pan drain the oil out of the differential, then check it with a magnetic (should be almost no metal). Opening the cover on the differential. Look at the wear pattern on the ring gear. Compare that to the service manual picture, should be wearing in the center of the tooth on both side. You could also remove the drive shaft and the axles (this would unload the ring and pinion). Then rotate the yoke on the differential, should be very smooth operation. Depending on the ring and pinion wear and how much metal is in the oil, you might only have to readjust the backlash or at worst replace the ring and pinion with new bearings and seals.

Re: Drivetrain noise
 

Not sure and all this guessing is not helping you, I realize that. I will stick with guesses, theories and the fact that it needs to get worse to find. BUT............. I offer another bit to ponder:
The slip yoke has splines, kind of like gear teeth. The ring and pinion do have gear teeth. The trans has gear teeth. In general when these are "driving" they are in tight mesh, so showing them clunking back and forth in static mode does not mean that the "looseness" is making the noise. Probably they are not "loose" during torque. The ring and pinion teeth are cut to not make noise like a spur gear does, but they can make that noise if the mesh or the backlash it wrong due to wear. That noise can translate to the front cab area. But who knows? It is still a mystery as to gear or bearing noise or vibration. I did not notice if you did a series of coasting tests yet. With clutch pedal depressed. Coasting is a good diagnostic because there is no drive torque on gears and clutch is disengaged. Sometimes it may give clues. More experimentation is advised.

The questions are is this speed related? what gear ratio it's in at the time? Does clutch in and out change anything? Does coasting give a clue? What area is the noise actually coming from? Is it gear or tire noise? (it can be tire noise. it can be wheel bearing noise, it can be a clutch fan if you have one). Is it vibration or mechanical rotation noise? Who knows? So there is your really helpful guess again:nothingtoadd:

Re: Drivetrain noise
 

Alright, will do and check the gears etc.

Re: Drivetrain noise
 

Ken: I have done some testing with the clutch pedal but that is not helpful as I only get it when accelerating at low RPMs. Read my previous posts.

Coasting, will do more experiments.

The reason for checking the different plays, as I did, is because the engine does not deliver a constant rotation. As each cylinder fires there is a 'shock', which is multiplied by crankshaft twist. The flywheel takes some of that shocking out of it but only to a degree.

My theory is that this contributes to vibration, as the vehicle speed translates to a constant motion while the engine produces the shock effect. This should fall away as the speed increases.

Maybe far-fetched?

Re: Drivetrain noise
 

Just for kicks, check the motor mounts and trans mounts. Bad rubber, broken brackets, loose, etc.

Your first post is confusing. I believe you said later on in the thread that the noise is after shifting into 4th.
But in the first post you say:

"When the RPMs drop (ex. from 3rd to 4th gear) you can hear the loud humming/grinding noise. It can be duplicated in all gears. Above approx. 1000 rpm it fades away."

So it can happen (duplicated) in all gears when RPM drops but "ex" = except from 3rd to 4th. But it happens in 4th.

This is hard to decipher. We only have the words description. The video sound is of no help.

Many folks may say it's synchros worn. But you say both trans had the same sound at the same time so I guess we are eliminating the trans in our minds. Not sure that is a good idea. What if it's two bad transmissions? Can you check the synchros in the other?

I will have to give up on this one. Wish I was there to hear it. You cant start tearing into everything. You can take it to a good mechanic to hear and/or fix those "loose" drivetrain components you describe.

Re: Drivetrain noise
 

I give up too. Thanks for giving some valid points though and if I ever discover I'll be sure to update this thread.

Re: Drivetrain noise
 

Drop the drive shaft and give her a stationary road trip - put a friend under the truck and listen. :thumbsup:

Re: Drivetrain noise
 

great idea :rowdybeer:

Re: Drivetrain noise
 

Its an old thread, but I think I figured it out... Turns out the steel wire clip that secures the clutch fork onto the bell housing clutch pivot ball was missing. With the new engine, I've installed a new clip.

I think this caused the noise due to the fork vibrating at low RPM. The clutch adjustment kept the fork from coming off. When you touched the clutch pedal the noise went away.