vetteguy75

Ok...I have a loud knocking noise in the front part of the engine that comes and goes randomly. I have replaced with new....the (1) Water Pump (2) Alternator (3) Harmonic Balancer (4) Mechanical Fuel Pump (5) A/C compressor...I have also checked the clearance between the pulleys (crank shaft and water pump)....

Now here comes the wierd part.....whenever I loosen the old A/C compressor and move it in (loosen the belt tension), the noise/knocking disapears. The first time I did that, I (along with my mechanic) thought it was the bearing in the A/C compressor...so I bought a new and, had it installed and THE SAME NOISE/KNOCKING surfaced!!!! I loosen the NEW A/C compressor & move it in...the noise disappears!!!! This knocking noise comes and goes...loud and soft..cannot figure what it is! HELP!!!!!

There's no oil leaks, the oil pressure is 40psi, the coolant is perfect...no loss....the oil level is good (1000 miles per qt)...the engine runs strong and true....JUST THAT DANG KNOCKING NOISE....

big_G

Check the crank pulley for cracks..

greg454

I had the same thing a few years ago and it was the water pump bearing.
Greg.

jclit

I had the same type of thing on a van years ago and after replacing the pump and alt. it turned out to be the belt. I don't know why but it made a metalic clicking noise. I new belt fixed it.
Jim

greg454

your best bet would be to track it down with an automotive stethoscope. Of course its not always easy to do with intermittant noises. Good luck.

vetteguy75

One of the mechanics at the dealership said it could possibly be the crankshaft bearing? He wasn't sure...but he said "that's all that's left"

Any comments?

Roadster71

I agree, try a new belt. The same happened to me on one of my trucks.

vetteguy75

It has all new belts.....still the knocking noise....

Jughead

Does the bracket that holds the AC compressor bolt to the exhaust manifold?

vetteguy75

Yes it does....

Jughead

I'm wondering if the manifold is cracked there. Does it bolt to the manifold bolt that attaches to the head? Maybe the bolt that screws into the head is loose or stripped.

Exhaust manifold leaks may cause a ticking sound.

vetteguy75

Thanks I will check it out....but the knocking sounds like it is coming from the front part of the engine.....

PhotoVette1

I had the exact same thing on a newish motor. The noise eventually went away. I suspect it was something floating around in the coolant that would get caught in the water pump from time to time.

MYBAD79

I had a similar problem on my '99 Z28 - the serpentine belt slipped a little and made a noise that sounded like a worn pulley bearing. I could have sworn it's a metallic clicking noise

I replaced the belt after I could not find anything wrong with the pulleys and the belt tensioner, the noise was still there

then I bought a better belt (these sepentine belts are $12 - $40 depending on manufacturer) and the noise is gone

Try some BeltDressing (STP) - spray it on the belt when it makes that noise. If the noise changes or goes away then you know it's the belt slipping.

wills670

I agree with the mechanic you talked with. I have heard one engine with a bad ft. bearing but it was a ford. I have never heard a chevy do this but that does not mean it can't happen.

I had a engine from a chevy make a knocking noise once and drove me crazy. Turned out to be a bad fuel pump but I see you replaced that all ready.

bryan717

Something you may want to check that would probly not be related to the AC compressor, but wouldnt hurt to check. On my 73 454 I had one of those elusive tapping noises comming from the front of the motor as well, it turned out to be a worn cam button, and bad cam. My cam was sliding in and out a little tapping on the inside of timming chain cover under the water pump. You may be able to use one of those auto stethascopes and touch that timing chain cover and give a listen. I picked up one of them auto stethascopes from auto zone for cheep.

JoeRags

used one to diagnose a bad balancer.... took two seconds to find exactly where the noise was coming from.

Solid LT1

Pull your fuel pump off and check the push rod. Something no one has brought up so far.

ZL1powr

I agree with big_G take a hard look at your crank pulley for cracks radiating from the three atachment bolt holes. Mine made the same type of noise at the front of the engine and it turned out to be the pulley. I replaced it with a cast iron L-88 pulley.

seaboltm

on my 73 the fan shroud was a tad loose. sometimes the fan blades would barely hit the shroud in one location only... tic, tic, tic,

revving the motor would shift the shroud, noise gone.

was pretty easy to diagnose though, so this may not be your problem.