E39 A/C Compressor noise

[NickC]

I have a '97 E39 saloon, it's a great car and gives me no real problems but I have had now, for about 2 years, a noise coming from the A/C compressor. I know it's this because if I turn off the A/C it stops.
It sounds like the bearing is sh@gged on the driveshaft of the comperssor yet I've had it checked out and it appears fine. The A/C works no problem and it's no issue other than it doesn't sound particularly impressive when sitting on tickover.
I tried spraying WD40 onto the pulley/shaft assembly and the noise went but then it's back a while later when things hot up under operation. Does anyone have any ideas as to what I could do to stop it other than the obvious replace the unit suggestion? Maybe something more viscous than WD40 that will stick and dampen the noise more?