I have a fender frontman r212 100w and it always makes a buzzing sound when the overdrive is on, I know it's not my guitar because I've played it through other amps and it doesn't do it.. but when I plug it into my amp it just sounds like a bassy buzz... Any ideas what it could be?
That's the way the overdrive works, unless you have a noise gate its going to make that sound.
It could still be your guitar. Single coil guitars in a electrically noisy environment can cause the problem you describe. Computers, Motors (like a refrigerator), and Fluorescent lights can cause noise.
Quote by JNBloomy
I've played on the same model amp at college and it doesn't make the same sound...

Like I said, overdrive causes noise, infact, a lot of things cause unwanted noise. If you have a friend who has a noise gate, try it out. If that doesn't solve the problem then look into your guitar like the others are stating.

Or it could be a power interference problem.
if your guitar has single coils anything from your monitor to the ceiling fan can cause noise.
1978 Peavey T-40 -> Ampeg Micro-VR - > Ampeg SVT210AV + Ampeg SVT-15E
It could be anything giving noise interference.

Take your guitar and amp into a different room, the "quietest" (least electrical) room if you can, and see if that changes anything.

Then grab some humbuckers, and new amp and a noise gate. Sorted!
call me ziggy.