1993 Grand Am knocking...bearing knock?

[JerkofallTrades]

It's a 93 3.3l V-6. This is my friend's car, and I was invited over to listen to a strange noise that was sufficiently frightening that I leaned way back, expecting parts to come flying out.

It's a mechanical knock, like someone is irregularly banging a hammer on the block. It seems to be roughly centered over the alternator/intake plenum, but I can hear it above and under the engine. It occurs at idle, disappearing when the engine moves above 1000 or so rpms. It's not timed to the engine, and is 1-5 hits per second.

Initially I removed the serpentine, and was unable to reproduce the noise by hand spinning the alternator, (a 1 year old rebuilt) the P/S pump, the A/C compressor, the water pump, or the tensioner pulley. I also spun each to a short belt powered by a hand drill, and at speed with tension I could detect no noise. I took the alternator to the local parts store where it tested good with no noise. I started the engine without the belt/accessories attached, and it idled quietly. I decided the problem must be the water pump, so I changed it. (fun? no.) The noise persists. It seemed like it went away when I engaged the A/C, but only was for a minute or two before it returned.

So my problem is this: If it's an internal problem (#1 bearing knock, oil pump, pushrods, etc.) why does the noise go away by elevating the rpms above idle or running with no accessories? If it's an accessory, what the heck is it? Does engaging the A/C elevate the idle rpm's up out of the noise zone? I also noticed the A/C pulley seemed to be ever so slightly forward of the belt centerline, but I can't see how I could get this kind of noise from that.

I have only located one motor mount (adjacent the accessories, removed to replace the wp.) and wonder if there is another (perhaps in front of the engine) that might be broken, but I haven't found anything obvious yet.

UPDATE: In trying to determine whether the dog bone was the rattle source, I took off the lower side panel for better viewing. What I saw was the harmonic balancer, with a nice tear in the rubber all the way around. So it seems clear this needs to be replaced, but now my questions are these:

Do you think the HB itself is the noise source, or just the cause of whatever damage is making the noise? (cracked crank, etc.) I ask 'cause I don't want to buy a new HB for $100 and then have the noise remain. This would be especially bad since any other likely cause would put the motor in the trash.

Also, I've read the procedure for the HB R&R, and can't determine for sure whether I'm going to have a problem with the crank sensor needing to be realigned, or what? It seems like the woodruff key would keep me honest on the new HB, but I don't know enough to be sure.

I throw myself upon the mercy of my betters. Thanks in advance for your advice.

[malletslinger]

Well the HB may be causing the problem...or maybe the torn rubber is hitting something as it spins?

The best way to figure out if something is causing a problem is to fix it then see if the symptoms go away... I would go to a pick-a-part salvage yard and pick up a HB there, then put that on and try that out($30 at most), if that wasnt the cause then at least you have eliminated a possibility and fixed a serious problem at the same time.

Or if that isnt an option, then take some additional things off and check for other damaged parts. I have a suggestion....
I suggest you check the timming chain, tensioner/guide, and sprockets...they can make quite a bit of noise, especially if the chain is loose...
If it aint the chain, then I would check the flywheel...I had mine break on my camaro one time and the car still drove find and the engine seemed to work fine too, but sounded just like you described as it was banging around(only at low rpms!)
BTW...How many miles are on the engine?

[JerkofallTrades]

Sorry about leaving that out: 95,000 miles.

As nearly as I can tell, there has been no other major engine work done to the car. The serpentine was replaced last year along with the alternator, but that seems to be it. I can't see anything in contact with the HB while rotating, but through the tear I swear I see loose parts inside. Are there internal pieces to a HB?

I dread the idea of timing chain inspection, as it seems like taking this engine apart in the car is painful at best. How hard is this inspection to do?

It's really being prepped as a "Baja Car", to be parked in San Diego for sprints south to the ocean cabin in Ensenada about 4 times a year.

I appreciate the suggestions and insight, as this isn't my car and I know less than nothing about this drivetrain.

Thanks again.

[malletslinger]

Well, to be honest, I am not familiar with this drivetrain either, but I did stay at a holiday-in-express last night.

So having said that, I think you should just have a mechanic take a look at it...Couldn't hurt...a good mechanic could probably tell you in a matter of minutes whether or not the engine would be worth repairing. They are good at recognizing sounds like that...I may have even been able to give you an idea of where to start if I knew more about the car and could see/hear it in person...