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Since the day I got my 336 back in August of last year it's had an occasional "clunk" that seems to come from under the front seat area.

Just had the roof repainted and while we were at it, softened the over-arched rear springs by taking a few leaves out. Rides and drives immensely better now. Today, after a week of shakedown cruises about 10 miles each, it started making the dull clunk sound with an annoying regularity and I'm going nuts trying to find it. Happens when you let off the gas when cruising down the road, happens when you finally reach full disengage on the clutch when backing up. Once in a while going forward but not often.

Can't duplicate it with the car sitting still. Put my wife in the car, jerked on the driveshaft, it has a little bit of slack in it but doesn't make the clunking noise, nor does the rear end. Something in the transmission? Weird mount problem somewhere?

Help! I'm scared to drive it any distance until I get this figured out! Thanks!

I don't have that model, but had a similar issue in an old Ford. The rear trans mount had separated--metal from rubber. Not so you'd see it naturally, but when I put a jack under the tailshaft and lifted it just a bit, the problem was obvious. You might look to see if you have something similar.

Also, despite the lack of movement in the driveshaft, how old are the U-joints? I changed the ones in my '37 115c as the rear had noticeable movement in it and was obviously "done", but the front U-joint was just as bad, and missing half of the needle bearings, while not showing any movement when idle. The grease inside the cups was also solid.

Magically that weird vibration disappeared when I got it all back together.

U-joints were presumably redone when they did the ground-up on her back in 1999/2000. Then it wasn't driven much at all, only about 320 miles until I got it in August of 2021 and I've already put over 500 miles on it.

There's no vibration or shimmy at all, it's very smooth at speed. Transmission mount is high on my personal list of suspects as well. Thanks for the input, will hopefully get the time to figure it out this coming Saturday, but appreciate all ideas of things to look for.

The engine is bolted directly to the frame therefore no rear transmission mount. I have been wrong before so check to make sure. I would check bolts on drive shaft flange and U-joints, also check differential pinion flange. good luck.

When looking at those things on the engineering side we glue piezo electric crystals on things and record the data.

Obviously, that is out of reach, so I would pull up the lower floorboard and have an assistant use an automotive stethoscope and look for movement in the driveline. The worst case scenario is a thrust problem.

Who suggested taking leafs out of the springs ? Not normal that I have ever heard of. Would that not change the angle of the drive shaft ? and cause pressure to be aligned differently on the transmission than as originally designed ? Did it do this before the leafs were removed ?

If a car has had a body swap from sedan to open car the spring rates will be too harsh and it will ride like crap.

If the springs were re-arched the ride height can be to high. When the springs sag the working angles move toward zero and if the spline is bottoming that could make a noise.

If the lubricator is not working that also will cause a harsh ride. I had people literally jump for joy over the change in ride quality when I fixed that system.

Taking leaves out is not my way of doing things, but they always can be put back in when the car sags.

As stated in the initial post, the "clunk" was there occasionally when I got the car last August. It has been driven very little since it was restored in 2000, only 320 miles in 22 years, I've put over 500 miles on it since August.

They extremely over-arched the rear springs to the point of making them useless during the restoration. They are the original springs, the car is not rebodied, they just went overboard with the rear springs - so much so you could stand on the rear bumper and bounce up and down - actually two people could, and the rear of the car didn't move up or down at all. It was so rigid, when you hit a bump the entire back of the car would hop up and bounce over, it drove terrible, rode terrible, and it was downright dangerous to drive on a rough road.

We took four of the ten leaves out on each side and that set the car fairly level, and she rides and drives infinitely better now. And the Bijour (sure I spelled that wrong), works fine. Collector Tom Brace up in Minnesota had the car restored in 1999/2000, they did a ground-up on a 29,000 orig. mile car, it had been garaged all its life, have spoken with him about the car. The ridiculous paint job it had was the result of a miscue with the resto shop, but other than the paint and the springs, they did a fantastic job and thankfully it was well photo documented.

We've had horrible weather down here this week, tornadoes all around us yesterday, will probably be the weekend before I get the chance to look for the "clunk" again. Thanks much!

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Here's another pic showing how high they had the rear end sitting.

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Okay, the noise is more of a pop than a thud or clank, and it's definitely coming from the rear end. I'm thinking(and hoping) everyone who guessed rear U joint is correct. So who sells them for a '29??

There's no vibration and no shimmy, very little slack in the driveshaft when in neutral and not running, but had a friend standing alongside the car as I went up and down the driveway, and it's definitely occurring at the rear end. Will try U-joints first, but these are the weird enclosed ones, so where do you get these things? Thanks much!