Drivetrain noise

[Prowbar]

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.

[AZKen]

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?

[Prowbar]

Originally Posted by AZKen

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?

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.

[AZKen]

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.

[Prowbar]

Originally Posted by AZKen

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.

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...

[James]

Originally Posted by Prowbar

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

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.

[AZKen]

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.

[Prowbar]

Originally Posted by James

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.

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.

[Prowbar]

Originally Posted by AZKen

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.

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.

[Prowbar]

Originally Posted by James

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.

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