Just put on a new alternator on my '98 4.0l, and have noticed a humming noise when running it. It sounds like a low/dry power steering pump, but the fluid level is fine. Everything is put back exactly how it was before, minus the new alt. Doesn't seem to be driving any differently, other than it sounds like a howling pack of mosquitoes under the hood. Searches I've done have landed me at "dry power steering pump." Any help would be appreciated.

 

You may have a bad bearing in one of the idler or tensionor pulleys, or a bad bearing in the new alternator.

 

Could the bearing on the idler pulley be put on upside down? It seemed to be fine and moving free while we had the pulley off, so I'm not thinking it's bad. The alt. also was spinning freely, as it should.

 

The odds of another component failing at the exact time you replaced the alternator (while possible) is very low.

Get a mechanics stethoscope and place it on the alternator. That should tell the story.

Or if you still have your old alternator, reinstall it and see if the noise is gone.

But the bottom line is looking at the "cause and effect" theory, and looking at work performed the #1 suspect is your new alternator.

 

Okay, not to bump this thread, but the other day while riving, I noticed the noise was gone. Cool right? Parked it at Tractor Supply for a little while, and upon starting it again, the noise was back. This is the first time I've noticed the noise being intermittent. This is gonna sound like a dumb question, and I think I know the answer, but are there break-in periods on alternators that could be causing the noise?

 

Nope. But there are rebuilds with cheap bearings (often they don't even replace the rear bearing in a chain-parts-store type rebuild). See TJ's post above- that's what you need to do.