Another "rattle" post - drive train related

[lespaul]

Stock 78 SC with 915.

This one sounds like someone put about 30 bbs in a coffee can and set it on the metal floor of the car. Someone not familiar with the car may not likely notice it. Almost a buzzing rattle.

It is not engine related, because it does not exist at any RPM when the clutch is disengaged or the car is in neutral. Thus, I conclude it must be drive train related.

It only exists when in gear and decelerating -- even when I just lightly let up on the gas or am barely maintaining speed. Most pronounced in 2nd at between 3000 and 3500 rpm. Can detect it in 3rd and 1st. Never tried in 5th. As the RPM's decrease the sound decreases. Drive train must be engaged and decelerating to hear the noise. Disappears on even the most gentle acceleration and engagement of drive train.

I would try to record it, but I doubt you could hear it with the other engine noise.

Ideas?

[CCM911]

Check the Heat Shield on your Catalytic COnverter.

[lespaul]

Thanks. Gutted Cat and no heat shield. Definitely drive train related.

[universeman]

I'm assuming it's different from the high-gear/low-RPM/trying-to-accelerate sound which sounds (in my SC) more like BBs in a plastic cup?

[lespaul]

Yes. I know that sound and it is not that.

[sugarwood]

Is that what a failing wheel bearing sounds like?

[zippy_gg]

Check the CV joints bolts and re-torque.
Even if this was NOT the problem it is still a good thing to do!

[mussberger]

I had a noise that sounded like that. Would come and go. I could get always get it to stop for a while by coming to a stop and putting it in reverse. After tear down for a refresh we found the reverse idler shaft bearings were perished. Last one in didn't put them in correctly.

[SWS911]

Sounds like it could be a throw out or a mainshaft bearing issue.

[DaddyGlenn]

lets see...Check my thinking on this..

No noise unless there is zero or deceleration load on the transmission, right?

That means no noise when rolling in neutral with clutch engaged or in gear with clutch disengaged, right?

So only when torque is applied opposite of engine rotation.

If that is true, then we are limited to places that torque would be felt most directly:

• CV joints
• Wheel bearings
• Transmission/engine mounts
• Any number of things inside the transmission

The first three can be checked easily. If its not one of them, drain the transmission fluid and see if anything falls out that shouldn't.

[lespaul]

Thanks Glenn. Your protocol makes sense. I did recently replace the tranny and engine mounts so they may have loosened up. I will also check the other easy options.