Amp makes hissing sound! is it supposed to do that? ?

I have a new Peavey 6535 (new version of 6505) when I play with gain/distortion it make a hissing sound! At first I thought it was my cabinet but when I play with no gain it doesn't hiss! What do you guys think it is? I have a noise gate too but that doesn't matter because it happens when I play not when I pause. everything sounds super good but with an ocean sound behind it, lol.