As promised I received an estimate to replace the return ducting to take care of the noise problem. I accepted it and they came out the next week.
The noise problem is now taken care of and they found another problem. The original installer (Arizona TradeMasters - see separate review) put a 5 ton air handler in the crawl space for a 3 1/2 ton unit. That's apparently OK with Trane but they're supposed to set the air handler properly - so it moves air for a 3 1/2 ton unit, not a 5 ton unit. They didn't do that which contributed to the noise problem. Scottsdale Air reset the air handler and did the same for my other unit at no charge.
The work was done efficiently and professionally - I'm very happy with my service.