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100 hours ago, a group-owned PA28 Archer II Circa 1986, 180hp plane had its engine rebuilt after 2500 hours. There had been no problems with the aircraft during the 2500 hours.

Since the rebuild, the plane is extremely noisy between 2,200 and 2,350rpm with significant vibration.

The propeller, the timing and the compression have been checked. Does anyone have any ideas why the plane should suddenly be so noisy in the cruise range?

The noise is a low-frequency booming.

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  • $\begingroup$ Not enough information. Clattering? Howling? White noise? Frequency of the vibration? $\endgroup$
    – John K
    Sep 6, 2023 at 12:07
  • $\begingroup$ Thank you John K - I have edited my question to include this information. $\endgroup$
    – John B
    Sep 6, 2023 at 13:20
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    $\begingroup$ Did they change the muffler at all with the engine? Boomy noises at high RPM is usually more to do with the muffler than the engine itself. Almost impossible to diagnose from a description, what does a qualified mechanic say during a demo ride? $\endgroup$
    – Jamiec
    Sep 6, 2023 at 13:52
  • $\begingroup$ Engine mounts? Loose, overtightened, wrong type (if changed) may generate all kinds of weird resonances. And along the lines of @Jamiec the exhaust system mounts (if applicable) should also be checked. $\endgroup$
    – Jpe61
    Sep 6, 2023 at 14:39

1 Answer 1

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The propeller beat frequency and the exhaust pulse frequency of a 4 cylinder engine are both about 60-80 hz. At 2300 RPM, 76.6 Hz to be precise.

A booming noise is likely noise in that frequency range (as if it was just the propeller noise amplified).

So two routes of investigation I would follow are propeller and exhaust system.

I would try two things:

  1. Remove the prop and move it one bolt on the hub. You want the blade to be ideally around 10 o'clock with a cylinder near TDC, but there is some leeway to change position. It could just be a coupling of the prop beat and the exhaust pulse beat in some sort of resonant phase that is entered at that RPM range. Reclocking the prop one bolt would disrupt that.

  2. I'd inspect the exhaust and muffler for looseness, contact with structure or engine mounts etc. If the muffler can be shifted slightly in its brackets, maybe move it a bit.

Try it out after each change, so you know which thing made an improvement if you get results.

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    $\begingroup$ A belated and grateful thank you to those who answered my question. Very much appreciated. John B $\endgroup$
    – John B
    Oct 12, 2023 at 13:40
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    $\begingroup$ @JohnB did you get to the bottom of it. As another PA-28 driver it's always good to know $\endgroup$
    – Jamiec
    Oct 12, 2023 at 14:45

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