Squeal from engine when starting up!!!

Hi all,
When starting up my series 90 there is a squeal from engine which goes once run for a few minutes. Thought it was one of the drive belts but they seem ok and right tension. Was going to attach a media file but dont think they are compatible files.
Any thoughts appreiciated
many thanks

Drive belts
Water pump
Alternator

Thanks,
Suspect alternator does it need to be a replacement job or can it be overhauled?
Cheers