This is shot in the dark, but there are a lot of really experienced people here that offer a lot of good advice. I have a '53 Chrysler 331 with the M6 clunk-o-matic. The car runs and drives excellent, and it shifts great. Everything seems to be working well. I drive the car pretty much daily. Over the last few months the cars has started making a really funny clank clank clank noise, and it seems to be getting progressively worse. You can hear it loud and clear echoing off buildings downtown. I thought initially a fan shroud bolt had come loose and the fan was hitting the shroud. But nope, nothing is touching it. It sounds like something rattling around in a coffee can. I though maybe a lug nut came off in the hub cap? Nope. It makes the most noise when I let off the gas and coast. The car will make the noise in Low, or High range, in all gears. I even tried engaging the parking brake a little when the noise happened to see if it made a difference....nope. If I push the clutch it, the noise goes away! Its hard to tell, but it sounds like it is at wheel/driveshaft speed. There are not signs that anything it touching the brake drum or driveshaft. I even pulled the inspection plate off the bellhousing to see if there was anything going on, didn't see anything. The pinion in the rear axle seems tight, and the u joints DO have a little play. I've had lots of cars with u joints go bad, and they would "rumble" or whine a little bit at high speed. Never a loud clanking sound at lower speeds. I put the car up on jack stands and ran it in gear and tried to find the noise. It is VERY hard to get the car to make any noise when the wheels are off the ground. I could bump the gas, and sometimes while coasting it will clank clank clank. I began to suspect the transmission kick down governor, but I placed my hand on it, when the car is in gear, and I did not feel anything when the noise was present. Also as stated previously, the transmission works great. I would greatly appreciate any advice. I'm hoping someone here has had the same problem before. Thanks for reading!