I know the noise you mean  

This sometimes happens due to grounding issues with the soundboard, check the mounting screws for the soundboard, make sure they are tight, or even remove them one at a time and see if the level of the noise changes.

On my KISS, i found that the noise was fairly loud with both screws done up but if i removed one of the screws it was greatly reduced... sounds weird to say it i know but it DID help reduce the noise!! I can hear this sound on both KISS and 6M$M if i mess with the sound board mounting screws, though i didnt need to leave a screw out on 6M$M  

Another thought i had, being a ground related problem, could also be the main soundboard connector ground connections (which i didnt touch on KISS btw)