One thing I will say just for reference, but doesn't seem the case with yours is I have had a problem on my old mk4 where the tensioner was flapping like crazy. You would only here a chirp from the belt when on full look or as the engine loaded up.
I changed the tensioner and it went away for a day or so then came back. Finally found out that it was the alternator pulley. It has a slipper clutch mechanism built into it which had seized.