I think since I've had my rig (over a year) there has been a annoying squeek in it. I'm almost always the driver so it wasn't that bad for me. And, since I was always driving I couldn't hunt around for it's source since it wouldn't do it while parked. I finally got somebody else to drive and figured it out, at least verified it was the a/c unit. I took it apart yesterday and found the culprit. If you take the plastic panels off on the inside of the roof there are four LONG screws/bolts, one at each corner. These have about a 3/8th size head on them, black in color. The go through the metal trim ring on the inside of the roof and all the way up into the unit. One of them was about an inch loose. It was scraping on the metal ring when the unit would vibrate on the road. I took all four screws out (one at a time of course) and put blue locktite on them and tightened them down. Drove 220 miles and it's quiet! Yay!!! I sure wish I would have done that a long time ago.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio