Does anyone know what this noise could be?

[blue62]

Some things I forgot to add.

After you do the belt off test...before you replace the belt spin the idler pulley with your fingers.
See if it turns smooth... or if it has any play front to rear. Do the same with everything you can that the belt turns.
Alternator...air conditioner pump what ever you can reach and turn.
If the noise is gone with the belt off you may be able to "feel" the problem by turning..spinning the pully's

Also is it a standard transmission???
or an automatic (tiptronic)????
If it is a standard transmission get the noise going.
then while sitting still in neutral push the clutch in an hold it in.
Then push it in and let it out a few times.
Does the sound change when doing this????