STRANGE ENGINE NOISE

[Bill Robinson]

I’ll try to make this description short. The engine goes in a ’27 Tudor. Numbers match.
• The engine was rebuilt 11 years ago.
• Scat crank was installed, as well as a .280 cam, bored, new pistons, Z head, etc. nothing was spared. Transmission and rear-end were also totally rebuilt at that same time. After quite a few tours and lots of local driving- no breakdowns.
• It runs great and pulls hills better that any of my 4 other Model T’s.
NOW the PROBLEM-> The engine has a noise. Not a knock (not a hollow sound), but more of a rattle. When? When accelerating at speed in excess of 25mph or going up a hill.
• Over the years, 2 well known Model T mechanics have disassembled/reassembled and both had the same opinion “it’s not a knock- it’s detonation”.
• Recently, my friend John Z and I have checked shim clearances, end play on the crankshaft, camshaft, and generator. No endplay. I just installed a rebuilt generator. The noise is still there.
• A few days ago, John and I took the car for a ride after the above adjustments were made and the same noise is still there.
• John was driving and listening, and after a few miles on the way home, he came to a stop sign with a mild/slight grade at take-off. He took off and held the trans in low to approximately 25-27mph, which is unusual for him. Then we both looked at each other rather strangely, and we both grinned and said, “NO NOISE on this hill!”. After another 2 similar tests, there is no noise if running in low, but shift to high, it returns.
At this point we both came to the conclusion that the noise may be coming from the transmission and NOT the engine.
Have any of you experienced this problem? If so, how did you correct the problem?

[TXGOAT2]

I'd investigate the possibility of detonation. It could be caused by a plug firing out of time, perhaps due to a frayed timer wire, a hot running valve, an intake leak, or a head gasket issue, among other things. A thin spot in the head casting might lead to a steam pocket and a hot spot. Does the sound occur when the engine is cool, or just when it is fully warmed up? Does richening the mixture make any difference? Have you tried using 93 octane gas? A spark plug running hot could cause detonation. Is there any chance a plug could be touching the piston? If it's a mechanical sound, running 10W oil and pulling one plug wire at a time with the engine hot might reveal the source. A loose wrist pin or piston slap could cause a knock. A misaligned rod might or tapered crankpin might, also. Timing gear problem? Crack in frame, loose rivets, loose motor mounts.... ? Could the Scat crank be touching something under certain conditions?

[Erik Johnson]

Years ago, my dad's '17 touring suddenly developed a strange engine rattle.

It turned out that one of the transmission bands was extremely loose - for whatever reason the nut had backed its way off almost to the point where it was close to falling off completely. I can't remember exactly which band but I'm thinking it was the reverse because I don't recall shifting or braking was an issue when the noise suddenly developed (if the low or brake band were loose, I would have noticed it right away via operating the pedals while driving).