My epiphone valve junior amp is buzzing... or rather, whistling, on certain notes. It's hard to explain.

Sounds like something inside is resonating with lower notes, mostly low E string ones.

I know it's not my guitar (it also buzzed when using a friends) and not the cord (just got a new one). I just recently sent the amp back to epiphone/gibson and they apparently fixed nothing.

Gonna try using it as a head to my friends speaker cabinet to see if it's the speaker that's buzzing.

Any suggestions as to what it might be?
Really? I guess I'll know once I try my friends speaker cabinet. I just cant believe they didn't pick up on it when I sent it back.
I have the head and extension cab and mine doesn't make any noise except for what is expected.

I think the other poster is right, check your tubes (valves). I hear they can sound very strange when they are on the way out. Besides, you can try some differant tubes to tune the tone some