Thread: knocking then whistling

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Well my SBC 350 is making a knocking noise. I believe it is coming from the motor but if you put your ear on the fender you can hear like a rattle.

I tried adjusting the timing and when advancing the knocking would go w/ the RPM's and then it would soon turn into a whistling noise until I retarded it, then it would go back to knocking.

I do not know what this could be be, any help would be appreciated

Thanks

51 Chevy 350/350

Just a guess here, a vacuum leak causing a cylinder to lean out and run hot enough to detonate. You can find a vac leak, or at least narrow down the area by using a 3 or 4 foot piece of heater hose or something close to that size. (Section of the neigbours garden hose!) Hold one end up to your ear, and use the free end to listen for the whistle. The hose cuts down all the other noises that are going on, and concentrates on whats close to the open end.

I found a vacuum leak on my old bronco with a bottle of carb cleaner (or starting fluid? I forget now). Sprayed short bursts around the engine, when it got sucked into the leak, the motor would rev up.

Be careful to keep the burst short or it could get dangerous. File that under redneck mechanic.

starting fluid works great, too bad i get in trouble everytime i light a customers car on fire .... they made me get a smoke machine, works really good though.... even though cars on fire produce enough smoke as is

Ok tried the starting fluid and it did nothing and I also hooked a vacuum guage up to the manifold and am getting readings around 12 and it moves like it is scenario 11 on this web page http://www.secondchancegarage.com/public/186.cfm but from 12 to 11 to 13 I tried adjusting the timing but it still does it. Also its got an HEI ignition

Ok well I've got it narrowed down to it only starts to knock after the car has warmed up, during the warm up when it is cold, no knocking noise???

Broken flexplate?

Is it more like a mechanical knock or a preignition knock? Sound like one cylinder or many? Could it be a spun bearing? Is the oil clean? Any smoke? Look for any sparkles or water on the dipstick.

-Chris

Not sure what kind of knock, sounds maybe more like a mechanical??? Just did a check by pulling plug wires and none of them stop the knocking. The oil is perfect, no metal specs or anything and nice and clear, no smoke, just did an oil change about 2 or 3 weeks ago

Ok well its wierd, I just did another check by putting the car in gear and it seems as if the knocking goes away. I also had the car revved up to about 13-1400 RPM's while in gear and holding the brakes and it didn't come back, but as soon as it was put back into park, the knocking came back
I am stumped

The bolts on the flexplate to converter are tight but I just disconnected the belt to the alternator and water pump and it seemed not to make any noise, could an alternator make a knocking noise, I replaced the water pump not to long ago so I don't think it would be that???

The plot thickens... if it's alternator or water pump, I wouldn't expect the noise to go away in gear(?) What about motor mounts? I was with Denny and or Corvette64 with loose converter bolts or flex plate problem but apparently that's not it.

You can try free-spinning the alternator pulley with the belt off to see if it makes any noise... Can try the same with the water pump pulley too.

Ok well I took it off, started it no noise, put it back on, knocking, took it off again, no noise, put it back on, knocking.

I tried free spinning by hand and it doesn't feel like its binding anywhere, but if I shake it back and forth like to and from the alternator i can get a clicking noise, water pump makes no noise and I just replaced that not too long ago

Originally Posted by DennyW

A torque converter will do that also. Check your bolts, and the flex plate itself for any looseness, or a crack.

Ok just came out from under the car and the bolts are tight and the plate and converter have no play in them. From what I can see I can't see any cracks unless their small or I missed them.

My next question is isn't there a certain way the converter is supposed to be mounted to the flexplate?

If that was wrong could it cause this noise??