Annoying Whining Noise

[S13drifter84]

I had an old alternator that was making a whining noise. Changed it out as well got a new alternator belt. After driving around for a couple of weeks the whining noise came back. Went to my mechanic and he said that he doesn't want to tighten it any more than it is already. Could the problem be due to the cold weather i am in. I live up in Canada. Oh yeah, it goes away after i hit the revs past 3000 rpm.