I have a squeek comming from the rear (147 selespeed '03) - can't work out where from as whenever there's a technician in the car it behaves itself. Had the drop links replaced to solve another problem but didn't stop the squeek. From sitting the drives seat it sounds like its comming from the ns rear but sitting in the back it sounds like its comming from behid the drivers seat near the floor - weired. It happens over occasional bumps especially when breaking.
I've just had a similar and maddeningly annoying squeak sorted out at my dealer. As the car is out of warranty I went in fearing the worst, but it turned out to be nothing more than a slightly loose bracket holding the exhaust.