This sounds like a ground loop issue. The problem probably isn't coming from the RackBrute's DC supply, but from a grounding issue with whatever you have the RackBrute connected to. The fix is to consolidate all of your AC connections onto the same ground plane. Probably the best way to do this is to star-ground everything to a central point that has an AC ground, such as your mixer. To do this, simply connect hookup wire between all of your equipment chassis and get those connections back to your mixer, which may well have a ground post for this exact purpose.
The next possibility is that something internal in the RackBrute isn't properly connected, and you're getting DC onto the chassis from an improperly-connected bus cable. If the issue persists after establishing a star-ground, then I would suggest opening up the cab and checking the bus cables to see if everything is properly seated on the busboard headers. If this is the problem, I would suggest keeping the star-ground connected anyway, as a good unified ground also helps to reduce overall noise issues in your system.