Alternator re-install humming sound
 

I just took out my alternator to have the VR replaced and just now got it back in. When I started it up, I noticed a low pitched humming sound that wasn't there before. I doubt a new VR would cause that. Is it possible that the alternator might not be alligned properly? If so, how do I check that?
Thanks
its an 87 Targa

I take it you spun the fan by hand, prior to installing the belt? I've had them (alternator) not seat correctly. If your blades were coming in contact with the housing, it would make a different noise than a hum, I believe. I've also seen powder coating that decreased the fan-tip to housing tolerance as well.

You might need to adjust the shims on the belt to tighten it up a bit more? I don't believe it matters what orientation the alternator is in as long as the bolts go through the fan housing and the cooling duct and everthing lines up that way.

I just replaced my alternator (bad bearings) last week and had the same humming but at highter rpm. Assumed it was the new belt and it went away within a few days. It didn't sound expensive but just strange. All is well now!