Classic/Antique Car Repair: 70 Chevelle, muffler shop, clunk

Question
Hello,
I own a 1970 Chevelle and it has a 350 engine, automatic transmission. It runs and ive had it for a little more than a year. It makes a loud "clunk" each time i put it in reverse. To get rid of the noise i took it to a mechanic to fix the u-joints, and the noise went away for a few days,but then it came back again. Now im not sure what it could be, by the way the u-joints were in really bad condition. I would like to know,if you have any idea what it might be,what i should do next or maybe what you recommend i check?  

Answer
There are two things that can cause this noise.  One is looseness in the drive line, and that is usually bad "U" joints, but it can also be a problem in the rear end or in the spring, engine, transmssion or shock mountings.  

The other major cause is an idle speed too high, so that when you put it in gear, it really shocks the driveline as the transmission bangs into gear.

If your idle speed is normal, and you still hear the noise, I'd take it back to the shop that did the "U" joints and ask them to check their work. The fact that the noise went away and then came back is very suspicious.

If they can't find anything wrong with their work, ask them to listen to the noise themselves and see what they think it is.

You may have to put the car on a muffler shop hoist (one that leaves the weight of the car on it's wheels) to be able to diagnose this.  Someone will have to sit in the car with the engine running and someone else will have to be under the car listening for where the clunk is coming from when you put it in gear.  It is possible that the motor mounts are bad, or that there is some excessive motion from the exhaust system - the only way to find a noise like this is to put it up in the air and demonstrate it to someone who knows where to look.

Wish I could be more help,

Dick