[fenbach]

to answer your question, try pulling out the antenna wire plug to see if that stops the buzzing. wishful thinking, perhaps, but a simple test. or maybe that’s what you meant by Not an antenna problem.
here's a link to information about the radio. maybe too much. with lots of "tests to isolate trouble." most of which require test equipment and that you remove the radio from the car.

http://www.nostalgiaair.org/PagesByModel...013846.pdf

good luck. and keep us posted.