if the amp is quiet when there is nothing plugged in and noisy when the guitar is plugged in I would suspect either a poor ground on the guitar (maybe the bridge ground has come off or opened) or it could be a bad cable (open ground or intermitant contact).
Have had both happen to me.
ALso, try plugging in a different guitar and/or with a different cable. You should know pretty quick what it is.
Here is a TRUE story.... I built a Trinity 5E3 Deluxe for a guy whop used to work in a local music store. He loved mine, and I did a really good job on his.... and made sure before I delivered it that it was working fine, which it was. I took it to the shop while he was working, and there were some guys in there. He grabbed a guitar (brand new) off of the wall hanger and plugged it into the amp. Turned it on, only to get the worst popping and crackling - while you could hear the guitar, I felt about 1/2" tall and was thinking "oh boy, now all these giys are going to think I cant wire an amp up". After farting around with the amp for a few minutes he decided to grab a 2nd new guitar from the wal and Voila! Problem solved. It was a bad ground on the imported Peavy guitar that was the issue/ or a bad pickup. But it was NOT the amp! I felt great, got paid and went on my way
AJC