Noisy serpentine belt tensioner - PD 150

I have a Mk IV 150 bhp PD Golf GT TDI with a worrying noise. The car is a 2003 model, has not been modified in any way (aside from 2 alpine subs and amps) and has done 82,000 miles. The car has climate control.

The noise was first apparent when manoeuvring to park at idle. I lifted the bonnet with the engine running and it was obvious the noise was coming from somewhere near the alternator. I noticed the serpentine drive belt tensioner jumping around alot, moving back and forth rapidly against the belt.
When revs are increased slightly, this rapid movement of the tensioner stops, the tensioner settles down in one place and the noise stops. I think the noise is caused by the belt vibrating and slapping back and forward.

Initially i thought the idler bearing was failing, so I ordered a new belt and tensioner from my local VW service agent, where i purchased the car from. Incidentially when i purchased the car at 61,000 miles a year ago I requested that the cam belt and tensioners be replaced within the purchase price.
I fitted the new tensioner and belt without any problems, and noticed that the spring tension in the new part was considerably stronger than the tension in the old part.

I started the engine and all seemed well, the noise had stopped. However, the noise has reappeared, albeit not as pronounced as before. It is only noticable inside the car when pulling away, at approx. 1600rpm. As revs increase it disappears. It is also possible to make the noise occur, by letting the clutch out with the brakes applied and making the engine labour, and this is when is occurs. The belt and tensioner can be seen to be jumping around as before.

Any ideas?

Thanks,

Nath