Radio problems

I'd run a test ground wire from the neg batt terminal to your radio case... You don't mention what vehicle, but a lotta these vehicles have a rear support/ground on the radio,, that often is not secured after the radio is removed/replaced,, and tries to ground thru the 2 screws on the faceplate..

If that didn't cure the issue,, and the noise goes away with the the alt. field disconnected, it's likely an old points style (2 wire) regulator,, and the noise is likely from the points opening/closing.. I think I would replace the voltage reg with a transistor unit..

But do the ground test 1st.. this is sooo often overlooked

hope it helps