Need help troubleshooting driveline noise

[Slartibartfaster]

My '79 has recently developed a knocking/clicking from somewhere under the car. It's happening at low and high speeds, but goes away with any throttle application. Power application makes it go away, it comes back the instant I back off. There is no change turning vs going straight.

For background, this is a 23k mile car, well maintained, solid, I have 39yrs of mx receipts. I recently did a carb/manifold swap, replaced the cat and switched to 14" biturbo wheels from stock "13 steelies. I have not yet replaced the flex coupling, but inspection shows no issues (I know you can't really see cracks at rest).

Here's what I've found under the car:
- PO was a believer in 'Italian Rust Proofing' so there's a decent coating of various lubricants on the trans and differential. Diff is oily, but the fluid level is just below the bottom of the fill hole.
- Car is on stands - with car in gear the driveshaft has about 1/8" rotational play, no for/aft movement, at the diff the shaft has no vertical or side to side play.
- Transmission support seems solid.
- With trans in neutral the drive shaft turns by hand with no clicking from u-joints (which are original)
- The center pillow block is original as far as I know, the bearing seems to move in around too freely in it's rubber support - but I don't know how firm this is supposed to be.
- About 100 miles ago I noticed the zerk fitting just aft of the flex coupling had thrown out some grease - I removed and cleaned the zerk and pumped in some new grease.
The driveshaft has uneven layers of paint/rustproofing over-spray. The welded on counterweights are still there.

Would love any advice on what this might be, and what I can eliminate - and how to figure out which is which.

[grrrdot]

Can you pinpoint where your hearing the sound from?
I’ve had the a failing driveshaft support create sounds that came and went under load but I would describe it more like a thumping then a clicking.
I also had good results with strapping a go-pro to the underside of the car to see what was going on.

-G

[MattVAS]

Take a very close look at your trailing arm bushings. These fail all the time and when you aren't giving it gas they like to knock. When you give it gas you pull the bolt firm against the trailing arm bushing hole. But while off the gas the rear end moves around thus causing the knocking as the bolt knocks back and forth in the bushing hole.

Hope that helps!

[Slartibartfaster]

Thanks both. I pulled the driveshaft (still sore) and I think the center bearing/cushion block has reached the end of it's useful life. It was pretty easy to move the shaft up and down in the block so this may be the problem. I ordered the driveshaft 'rebuild kit' from Vicks to replace the guibo and u-joints while it's out anyway. Think I'll find a shop to do the dirty work on the shaft then get it back in. I did clean off 39 years of collected oils and fluids - not rustproofing over-spray.

I did not think of the trailing arm bushings...just checked and they seem firm and the trailing arm isn't moving, but the car is still on stands with the rear suspension not hanging, but definitely extended. I didn't think it was a great idea to leave the suspension in full droop while the car's in the air for a couple of weeks so I jacked the diff up a bit to keep some weight on the springs. I'll check the bushings again when I get it back on the ground.

[Slartibartfaster]

Fixed! New ujoints, center block and bushing and flex disc - noise gone.

[TwinFast]

Fixed! New ujoints, center block and bushing and flex disc - noise gone.

For me it was a different story :
The driveshaft of my 78 Fiat was noisy. I put new U joints, flex block and bushing - and the noise become louder ... The wheel bearings are new and the front and rear differential pignon bearings have less than 500 miles, except the 2 carrier bearings which have not be changed for a while.

Could these 2 carrier bearings be more noisy when the other bearings are changed ?
I also suspect a poorly balanced transmission shaft after servicing ?

Any idea ?

[Slartibartfaster]

That's beyond my ability I'm afraid. Anyone?...

[78Blue124Spider]

This is very late in coming but I had the exact same issues. I chased the problem from clutch to transmission to drive line to couplings to finally the "final drive" assembly. The differential was TOAST. As soon as I changed the differential the noise went away completely. I opened up the housing and all the gears were shattered.