Karelfd, the wall wart in the MPC is just a garden variety 300mA 24V transformer that you can buy for about $15 online. I've had a couple of them burn out, and after once buying the real deal replacement for an idiotic price, I just replace them by cutting the wire and splicing in a new transformer. So, perhaps you can try a replacement transformer and see if that stops the noise.
I don't think the little pyramid sticker has an audible effect on the sound!
I don't think the little pyramid sticker has an audible effect on the sound!