If you have a noisy alternator its probably either the front bearing or the overunning pulley.
To check if the pulley is ok you can do the following procedure.
Remove drivebelt with 14mm tensioner tool or very long 14mm spanner.
Remove cover from end of pulley.
Using suitable allen key (or if like me you didn't have one suitable hex headed bolt and molegrips) hold end of pulley.
Turn pulley by hand both directions.
It should lock in one direction and not the other.
If it locks in one direction and is stiff in the other ( like mine was) or doesn't lock in either direction the pulley requires replacement.
if you are going to replace pulley you may as well do the front bearing as well.
Make sure your engine has the modified shorter belt as you don't want to have to do this all again in a few years.