Guitar processor noise...? BZZZZZZ!

You could test the ground by getting some wire and attach to a bare water pipe the cleaner and former the contact the better.

One way would be to Scotch Brite an area and then put a hose clamp on it tuck the wire between the pipe and the hose clamp and tighten.

Then take the other end and touch exposed metal of the devices starting at the farthest from the ac outlet.

If the buzz goes away you know there is a bad ground between them.

This assumes that your pipe is copper or iron and is tied to the house electrical ground.