M-400 Buzzing

My M-400 has developed a buzz in the amp and now I can hear the buzz through both speakers. Could someone who may have had/have the same condition have any idea what this may be?

I'm planning on sending this to Rita soon, maybe as soon as next week but it's buggin the hell out of me . I'm taking it off the rack today and putting my newlt restored, from Rita, M-400a back on. But, whenever a piece of my gear developes a problem, I like to know what it is. Thanks for any assistance or info
Ed

I had a buzz from an M-400 I bought through one channel. Rebuilding it did the trick. Never quite narrowed down what was the cause, but it went away after the rebuild.