Strange metallic noise of left hand side of engine

Hello, just gone and started my octavia (1.9Tdi GLx Estate 90bhp 1999 manual) started fine, but a couple of seconds after starting there is a strange metallic noise form the engine.. with the bonnet up it seems to be coming from around the alternator area.. How do I check to see if it's the alternator bearings have gone.. and is there anything else it could be..

Can you replace just the alternator bearings.. or is it just a case or buying a complete re-con'd unit?

Thanks for your help..

Dave