It's likely the cooling fan. It's supposed to run to keep the amp cool and most of them make some degree of noise. If your ears are perked up playing quietly in a dead studio, you'll hear it as will you when you first turn it on. If you're playing loud enough to keep up with a drummer and guitar player/keys/vocal/horn/distortion or whatever else, it's a non-issue.
If your hear the cooling fan come on, it means that no matter what other crazy stuff may happen that night, your amp overheating is not going to be an issue....you'll likely have plenty to deal with other than that.
