Pretty sure that is a grounding problem on your cassette deck. Most likley coming from some type of bad connection that will be hard to identify
I had that same problem on mine with my rc 656. I took the boombox appart down to taking out the cassette chassis and did a bit of basic cleaning. I then tried to find the bad connection, but i could NOT find out exactly what was wrong, but when i put the cassete deck back together the problem Magically stopped and never came back.