As Dave mentioned (and has a great memory, I might add!), it is an aftermarket radio.

From the information you're giving, it sounds like a wiring issue...either bad ground connections, or some crossed or crushed wires. In two different cars in the past I've had issues with weird things like dash lights going dim and each time it was wiring that had been crushed.

It may take time but check all your wiring and connections that lead to the dash lights, power windows, etc...anywhere they cross...even the printed circuits in the guage clusters if necessary.

i have this problem on my 73, not with the casset but with the radio and i didnt have the niose untill i put in my msd dis, & 6al ign, control

Mark...

Did you reinstall the shielding around the distributor and plug wires? Maybe the 6AL needs to be shielded as well. You're obviously getting ignition interference. Maybe better quality resistor wires could help, but Chevrolet installed the shielding around the distributor, coil and plug wires for a reason.    

I installed an MSD 6A in my '69...hidden behind the dash panel on the passenger side and have had no problem with noise. I have an Alpine CD paler/stereo installed...maybe it has better interference filtering built-in than what you have.

I don't know what make stereo you have, but I've heard a lot of complaints about Custom Auto Sound units for numerous reasons, and they seem to very popular units for older vettes and older cars in general.

my radio is from custom auto sound,maybe they sell some sort of a filter

I drove it to work today.  The buzz started almost immediately.  I did notice that the pitch changed as the RPMs changed.  HOWEVER, this was only true while MOVING.  If I was sitting still in Park or Netural and reved the engine, the buzz did not occur.  It only happens while moving.

[QUOTE=mark smith]my radio is from custom auto sound,maybe they sell some sort of a filter [/QUOTE] Redwingvette2006-11-27 08:11:34

[QUOTE=Adam Wartell] I drove it to work today. The buzz started almost immediately. I did notice that the pitch changed as the RPMs changed. HOWEVER, this was only true while MOVING. If I was sitting still in Park or Netural and reved the engine, the buzz did not occur. It only happens while moving.[/QUOTE]

Sounds like muffler bearings to me.

Adam, while "sitting still in Park or Neutral", did you have yer foot on da brake

  For Aftermarket Radios and Amps the C3 Corvettes are known for there Ground Loops. They have driven many an Installer wild. The Standard way to solve them is to Run all Your Grounds in the System to one Location Like on the Frame under the Battery. When this dosn't work then You start adding Noise Buffers to whatever is making the Noise. MSD Sells them for the 6AL. You may also add them to the Alternator and or anything else making Noise. On my High End System from the 80's I had them everywhere. After Ripping out all the old wireing and starting over I used Zapco Amps and there Sybil Link System and have very little Noise with No buffers at all.  Sounds like You have something amiss besides this though. I wouldn't get to wound up on the Spark Plug Wire Covers as any Modern Plug wires are so good at Supressing Noise the Sheilds really don't do anything any more anyway. Look at the late C3's with the Plastic Cover with a bit of Aluminum Foil, what was that going to do. Richard.....