Could be the tensioner pulley going bad under a load i.e. ac......just a thought. He could be right on the ac however it may be the pulley for the ac compressor instead of the ac itself. Get wd40 and start by sprating the tensioner pulley then spray the pulley of the ac. Dont actually spray the pulley or the belt would probably greet u with a slap to the face. Apply lube to the bolt area or rather the bearing cover to atleast rule out one of many pulleys that may squeal under load.
1996 Mustang GT, 17x9 front and 17x10.5 cobra r replica rims, 255/40/17 front 285/40/17 rear at the moment,
o/r x-pipe flow master 40 series dump downs, cai/ram intake, 75mm c&l throttle body and plenum, pi heads and intake swap, msd wires, stock gears, untuned at the moment, no dyno numbers for now.