Mystery noise (loud) from alternator? Not belt squeal...

[PDutro]

My 2005 5.3 4wd Tahoe just started making this noise when engine is started from cold. It started yesterday, I noticed it while driving through a parking lot (I had driven less than 1 minute from my house to the store before I heard it). I parked it and let it idle while I ran into the store for a coffee. But the time I returned it had stopped and never happened again that day while we drove and parked and drove and parked.

This morning I start it up with the hood up and it happens again. It's fairly loud, doesn't sound like belt squeal but more of a high pitched grinding sound. It comes and goes 10 or so times, lessening and finally stopping once the engine has idled for a bit (maybe 3-4 minutes). It almost seems like it's concurrent with a duty-cycle of some kind but I can't figure what it might be. Any thoughts? In the video you can hear the motor running and sounding normal, and after a few seconds you can hear one or two examples of the sound, lasting a second or two. Then it goes away and doesn't come back (I caught the last one before it warmed up enough to disappear) Thanks far any help.

 

[TTR_48]

To me it almost sounds like a dry bearing, do you have a mechanics stethoscope? Can you tell if it is coming from the front or the rear or center? I had a idler pully on my old Dakota that sounded like that when the bearing was starting to go. One way I tested it was using a bit of Machine oil on the bearing to see if it quieted down. If it is the alternator, there is a bearing in the front and rear.

 

[PDutro]

To me it almost sounds like a dry bearing, do you have a mechanics stethoscope? Can you tell if it is coming from the front or the rear or center? I had a idler pully on my old Dakota that sounded like that when the bearing was starting to go.

That's my thinking, too it just sounds different than other failing bearings I've heard. But you're saying you had one that sounded similar and came and went until it warmed up? If so that is helpful.

It sounds like it's coming from the back. I can't imagine what else it would be so I will probably throw a new alternator at it, though I am not a fan of throwing parts and hoping for a fix. But the tensioner pulley could use replacing too, so I may as well do that and a new belt while I'm at it.

Yes, I've used the stethoscope but the noise is loud enough and goes away quickly enough that it is tricky to track down anything definitive. I'll try again today and report back.

 

[TTR_48]

I used a product similar to this to oil while running, with its design it mad it easy to do. It's a simple way to troubleshoot and cheaper than a new alternator if it is not the issue.

http://m.homedepot.com/p/Super-Lube...colon-PTFE-Lubricant-Per-Each-51010/202932717

 

[PDutro]

I used a product similar to this to oil while running, with its design it mad it easy to do. It's a simple way to troubleshoot and cheaper than a new alternator if it is not the issue.

http://m.homedepot.com/p/Super-Lube...colon-PTFE-Lubricant-Per-Each-51010/202932717

Good thinking. And what did it take to get at the bearing to apply the lube? Did you have to remove the pulley or pull the alternator off?

 

[PDutro]

When I started it up from cold today the noise had changed and is now not as loud, but more constant and almost certainly coming from the alternator. Suspect failing bearing(s) as we thought. I have a replacement on it's way, hopefully mine will keep cranking until I can change it out! It's a Bosch, I believe it's the original alternator still, so it should be pretty robust and I'd like to think it will run for another few days at least.