Yes, a bad capacitor can cause this problem. So can a lot of other
things.

A capacitor which is arcing internally (dielectric breaking down) sounds
just like you described. Chances are it'll be a bypass or coupling cap.
Other things to try are; wiggling tubes (corroded sockets), look for
cracked resistors, cold solder joints.

Good luck!

Steve
p.s.-a bad filter cap usually causes hum which you'd hear in the speaker,
and possibly on your transmit audio....filter caps can also arc internally,
but
in my experience, thats not as common