I've been watching this thread because I had a similar thing happening with my GF-777. I am using the GF-777 as speakers for my computer. A mechanical "machine gun" sound started coming out of the speakers one day. I thought it was starting to die, at least one or more caps, so I just decided to live with it until I had no choice but to try and fix it.
Well, today the sound wasn't so prevalent, but then I noticed it would get louder then softer. So, I started working switches, knobs, everything. Pushing the left cassette keys made the sound quieter. Odd. I pushed the right cassette keys - no change. Touching the left grille made the sound quieter too. Twilight zone. So, thinking there's a ground issue with the entire left side, I ran my hand down the left side of the GF-777 while touching it. The sound disappeared.
Okay, I'm really flummoxed now. I remove my hand, sound comes back. Then, I had an idea of what the real problem was...
We had recently upgraded our Comcast xfinity modem and the modem was sitting - you guessed it - about six inches to the left of the GF-777. After moving the modem further away (about 18 inches now), there is no noise.
Electronic interference. Who would have thought? Never had an issue before that.