If Richard's and PDQ's tests exonerate the cables and input sockets, and it's a really loud "I can't play with this thing making this racket" harsh buzz, I'd bet on a power supply capacitor. Especially if it's an older, fairly high-powered amp; heat and age conk the things out with some regularity.
The good news is that is generally quite cheap to repair (if not done on an emergency basis). But also not something to mess with if you don't have some knowledge and/or good guidance.
-Glenn