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I've got about 2,000 miles on a rebuilt 1936 Pontiac 6 flathead motor. From the beginning, it has made an odd noise when revved in a certain rpm range. It is coming from the front of the engine, sounds like a knock. I know it isn't:

• a generator bearing;

• the water pump or fan;

• engine bearings, pistons or valves;

• the inner oil splash pan;

• anything loose on the car that might vibrate (gas line, etc.)

• the distributor.

I'm wondering if it might be either the harmonic balancer or the timing chain/gears. The balancer is original and worked fine before the rebuild. The new timing chain was new, but the timing gears are original (impossible to find NOS gears). I've never had a problem with either of these, so I don't know what they sound like when they are not functioning properly. Or maybe its the engine mounts?

Any suggestions/advice most welcome. Thanks.

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  • 3 weeks later...

Bob, is the knock a deep, solid sound similar to a rod knock? Typically chain driven camshafts don't knock like gear driven cams do because chains don't impart thrust; but it is possible the cam is "walking" in and out due to varying loads from the oil pump. Can you tell if the sound is loudest at the timing cover, or the engine block just above the cover? The most common engine to be plaqued with loud camshaft knocking was the 4 cylinder "Iron Duke" engines used in the '80s and early '90s in GM X & N body front wheel drive cars. They had gear driven cams and if the cam gear had excessive clearance the knocking could be heard 100 feet away!

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It's more of a "hollow knock" Pete, so the current theory is that I may have a bad piston. Compression check comes next, so we'll see. The rebuilder is some distance away, so haven't had it back to him. Thanks for weighing in everyone.

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  • 2 years later...

hmm..I have the same issue with my 27 pontiac...its like a slapping sound coming from the upper half of the motor.....the engine sat for 20 years...got her running last week...not sure if I should let her run and the sound is normal or be worried.....

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