Thanks for your reply :-)
The RackBrute is not connected to anything else!
I have tested the make noise skiff again, and it actually also exhibits this behavior, with only one module in it (with this module taken out of the RackBrute, the RackBrute feels buzzy). So unless it is multiple modules that causes this, it is not a module problem.
Further, if i touch the ring of the dc plug with one hand, withOUT it being connected to the RackBrute (so it is unpowered and not connected to anything) and touch the RackBrute with the other hand, is also buzzy feeling, as soon as I let go of the dc plug it stops