In my reply to your PM, I was thinking you had an inline Magdrive. Adding a baffled box around it won't help. What seems to be happening is the side of the sump is making the pump more audiable. It is acting like a band shell (what a band or orchestra plays in outdoors so that the audience can hear). I think the pump may be vibrating the water which is carried to the side of the sump, that is then creating the audiable sound. See if placing your hand on the side of the sump will quiet it.
Also go ahead with adding a section of hose to the return. Hard plumbing can facilitate in the transmition of vibrations.
Replace the two wash cloths with a piece of foam or a sponge to see if that helps.