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I am getting knocking sounds at low RPM. I originally thought a cover or shield was loose. The sound appears at initial start-up, than stops when the engine settles in. The sound will come back if the RPM is raised just above idle, but goes away at 1,200 (?) RPM.



The sound seems to come from around the alternator, or lower, area. When touching the alternator it feels like the bearings are shot. The alternator is not the problem, I swapped it with the one from my 951.



Any ideas?

Knocking ?! If it happens on deceleration of the revs as they drop from around 2500 rpm down to idle, check the oil pressure right away just to make sure it's not rod bearing knocking.. unlikely as that may be, better safe than sorry ..



I think alternator bearings would make more of a metallic rattle than a knocking sound, but you already replaced that one..



Lifters tapping / knocking usually happens when you just start it and resolves when the engine settles in and warms up a bit..



No other ideas.

Something simple like heat shield maybe?



Jay

@ ds968 - Oil pressure is good. The sound happens on acceleration between 1,000 and 1,500 RPM. it is not a tapping sound, more of a dull knocking.



@ 94SilverCab - I've looked around for a loose heat shield, haven't spotted one yet.



PS. I'm not driving the car until I figure it out.

Is it only when you start from a stand still, or are going very slow and then accelerate?

My reason for asking is, this is how my torque tube starting letting me know there was a problem with the bearing closest to the engine.

There is an inspection whole for the shaft, and in my case the articulating joint was moving under initial load and hitting the sides sometimes.

I'm assuming from your original post that this is when the car is not moving at all, and that even after swapping alternators, it still feels "like the bearings are shot"?



What is the tempo of the knocking...?

This happens when the car is moving or not moving, no difference is noise, so it is not related to movement.



The knocking sound is not rhythmic, it is rather random, like rocks rolling around in a can.

i'm betting that it is a broken ear on the air conditioning compressor - happens all the time when people adjust the belt but fail to properly loosen the mounting bolts first and then re-torque them

Maybe a balance belt popping until tensioned. These darn things require very little tolerance between loose enough not to whine and tight enough not to knock. I may be thinking a bit too far out of the box.

Problem solved. I have to give kudos to flash for pointing me in the right direction. While looking for a broken ear on the compressor, as flash suggested, I found the front compressor mounting bolt missing! Looking back, I should have looked into this. When I removed the belt tensioner the compressor moved rather easily.



Thanks flash!



Thank you all for your help.



Thom

no worries - happy to help



that's the "advantage" of having dealt with these for a while now - unfortunately the flip side of that particular coin means that i am constantly listening for weird noises and stuff, which tends to diminish the experience of driving - i often wish for more blissful ignorance



oh well - that's why they make cabernet

Quote: the flip side of that particular coin means that i am constantly listening for weird noises and stuff, which tends to diminish the experience of driving

Ain't *that* the truth... I call it "wuhwuzzAT?!" because that's the first thought I get every time I hear a new clunk, whirr, whine, buzz, rattle, screech, rub, or humpty-hump.

Some say that when he turns on the ignition an antenna extends from the top of his head, but all I know is his name is Flash! Yes his Flashness is as good as right on the the money with the bracket problem. I never fail to smile when I read of our man coming up with the goods time after time. Everyone bow there heads! Better still send Flash a case of Cabernet. No! Wait! We need him sober at least some of the time to help us mortals out of the doo da!



Andy

Some say that when he turns on the ignition an antenna extends from the top of his head, but all I know is his name is Flash! Yes his Flashness is as good as right on the the money with the bracket problem. I never fail to smile when I read of our man coming up with the goods time after time. Everyone bow there heads! Better still send Flash a case of Cabernet. No! Wait! We need him sober at least some of the time to help us mortals out of the doo da!



Andy

No that's not a double entendre, just a loose connection causing a problem!



Andy

Bob, hate to rain on your parade, but I am blissfully ignorant yet I still hear noises and worry!