I think it is simply a ground problem. I have had that several times with various pieces of equipment. Just get yourself a $2 adapter that has a 3 prong female on one end and a two prong male on the other. This will lift the ground. Put it on your source and I can bet your popping will disappear like magic!
You will probably get your Rotel back and still have the popping sound.
Arthur
You will probably get your Rotel back and still have the popping sound.
Arthur