Alternator Pulley

[rovertone]

Does anyone know whether the pulley on an 03 CDTi incorporates a one way clutch.
I ask this as I have a mysterious problem with an auxillary belt that bounces at tickover causing the belt tensioner to bounce which eminates a knocking sound, as soon as the revs rise a fraction all goes smooth and quiet.
I inherited this problem with the car and thought that i had found the cause when I rectified a wrongly fitted crank pulley (it was running 'buckled' due to not being correctly seated), at the same time I changed the tensioner. The knocking and belt flapping improved slightly but the 'flapping' moved further round the belt to just before the alternator. I have just come across a comment on a Mercedes forum where someone has said that if you experience this it will certainly be the pulley clutch that has failed but I cannot find any reference to any such clutch on R75.
Any advice or thoughts would be most welcome.
Thanks Tony