I recently bought a used Peavey 5150 II. I brought it home played it in my garage for about 30 minutes and it didn't have a problem. I put it in my room and now i get this rather large humming noise. The noise stops for a second when i tap on the head, but it always comes back. I turned my "resonance" and "presence" knobs to zero and it goes away for a while. I also tried putting the ground switch on "-" instead of 0, and the volume on the hum decreases a little but it's still there.
I've read that it could possibly that one of the screws that secures the preamp pc board to the chassis, but i dont know where those screws are.

Any help would be appreciated.
first thing I would try (especially since it worked fine in your garage)...would be to try a different outlet in your house. The outlet you're using could be the problem...in addition to other environmental things in the room (fluorescent lights, computer monitors, etc.), can all add noise. I would try this first before narrowing it down to being an amp problem.
Genz Benz El Diablo 60
Avatar cab with Man O War, Black Powder speakers
Schecter 20th Aniv. Stealth #07 out of 100
Now that you mention it, i noticed that my B-52 ST100 had a weird humming problem. I guess it doesnt help that i have a computer, a tv, and a external hardrive within 3 feet of my amp.

I put it in a different room and used a different wall outlet and now the major humming is gone. If i tap it a little it still barely hums for a few seconds. I dont know if thats a minor problem i might wanna look into.
Computer monitors, phones, lights, radios all affect your sound. My amp is very sensitive towards lights that you can turn the brightness down on.

Roger Waters - 12th May!