"harmonics" noise

A few years back, found a beauty...2005 Jamboree GT, 30 ft with only 2000 miles on it. Bought it from original owner and when driving home on freeway, thought someone was honking at me! Soon discovered it was coming from the motor home. Called the seller when I got home and he said it was coming from one of the shrouds or antennas on the roof. Informed him I was having my mechanic take a look. Checked wheel bearings, rear end, drive shaft, U joints, transmission and no one can figure out what the noise is. Happens between 50-65 mph, not as bad under load or coasting but definitely throttle related. Checked heat shields, installed Banks system, still the same noise. Any ideas???