I've been the owner of a 1978 (third gen) Monte Carlo for four months. It has an auto trans (TH350), 305 V8, and power everything. The only thing wrong with the car is a peculiar rattle which intermittently makes itself known while driving.

I'll describe it the best I can:

-seems to happen when the car is slowing down (braking, etc) when RPMs drop
-sounds like metal on metal, rapidly rattling
-happens intermittently, but one peal of rattling can go on for 10 seconds or more
-tends to happen when the wheel is straight, and especially while braking (as aforementioned)
-sounds like it's coming from the rear of the car, beneath the trunk
-there's no feedback through any of the controls, only the sound to indicate something is wrong

Everything under the car appears solid, so the source of the noise is a mystery to me. I thought it might have been the parking brake cable going slack and swaying, but it's not the case. I'd like to be able to fix the problem myself, avoiding a garage if possible.

Any information would be much appreciated. And if I've categorized this, please redirect me.

 

Could it be the exhaust system,muffler or tailpipe (I have a car that does this while idleing only in reverse). Also Had a loose flywheel before that made alot of noise when the rpm dropped. Just a few ideas off the top of my head maybe they'll help

 

The exhaust system is much newer than the car (dual flowmasters). It's tight enough not to rattle around - tailpipes included. Could something be rattling around inside of the muffler?