This thread references: http://www.airforums.com/forums/f450...reo-36282.html
After much fruitless discussion and trying a number of solutions, here is a success.
Strange noises are gone from the stereo and subwoofer on my 2007 Safari.
Some noises went away after I installed two noise filters on the power for the subwoofer and stereo (such as pump noise),
but the best result was a ground loop isolator on the subwoofer input.
No noise, even at very low volume.
My dealer mechanic recommended that, and even sent me one gratis.
You can get them at Radio Shack, about $20.
Seems grounding is not the Airstream's strong suite, even though it is