Spray both sides of the tensioner bearing with wd40 while the engine is idling.
If the bearing is at fault the noise will stop instantly.
have recently changed belts on my 300tdi discovery and now wont stop squealing have tried belt dressing and talcum powder which is ok for day or two then back again sounds like a steam train CHUFF CHUFF CHUFF CHUFF have been told i need to fit a GATES belt
Ta Battenberg, BUT it looks as if it's the alternater bearing, using my flat blade stethescope theres a hell of a rumble in the alternater body - can you get replacement bearings for alternaters?
Try the penny trick first. Costs...a penny and a few minutes