If there's no noise between stations with the muting off it's probably
oscillating. That would maybe indicate filters or bypass caps. If the
frequency actually drifts, assuming it has ONE fault which is usual,
look around the FM oscillator or mixer stages.

I think buzz between AM stations is just about normal these days,
unless you specifically noticed a difference at the same time the FM
fault showed up.

Actually RF is not really my strong suit, but it's still electronics.
If that reciever uses discrete transistors for the FM, it might be a
bit hard to find someone who really remembers how to work on it. That
reciever is old enough for a pension, but by all means try to keep it
alive.

J