Drivetrain noise

[James]

Originally Posted by Prowbar

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

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.

[Prowbar]

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.

[Prowbar]

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?

[James]

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.

[AZKen]

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

[Prowbar]

Originally Posted by James

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.

Alright, will do and check the gears etc.

[Prowbar]

Originally Posted by AZKen

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

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?

[AZKen]

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.

[Prowbar]

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

[lizziemeister'sV6]

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