I bought some "pipe tape". It is foam rubber with one sticky adhesive side and wrapped the solid parts of my return pump with that. That reduced about 80% or the noise. I then bought a roll of 12" wide, 2" thick fiberglass wrap that is used to insulate duct-work or wrap around hot water heaters and put that on the inside walls of my tank stand. Now all you hear is a quiet hum. My wife likes me again and the aquarium is whisper quiet. The protien skimmer pours out over rock rubble and the water return makes no noise at all.
All told, I spent $12.00 at Lowes for the fix.
Mike