howling noise

I have developed a howling noise from my 2lt pancake motor.It only happens at idle,as soon s you put more revs in it disappears.I put a screwdriver on the engine and found most noise occurs between fan and alternator.
Any ideas would be great.
cheers

bearings in the alternater

cracked fan

Thanks heaps for the help,its the alternator.started her up this morning and you could hear it as plain as day.Took fan belt off and spun alternator and sure enough the bearings gone.

there ya go all better