Looping sound when climbing/cruise power
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I've noticed a fairly strong/loud looping sound in the Arrow III under any kind of flight power with the exception of descent. It directly varies in speed with changes in RPM and loudness with changes in power. It is very noticeable with headphones and fairly noticeable with speakers.
The pattern sounds like you are driving over expansion joints on a long overpass/bridge. A sequence that is hard to describe in words but looks like the following:- (DU DUM <2 sec pause> DU DUM <2 sec pause DU dum) <end of seq for 4 sec> (Repeats)
The last one in the sequence fades away. This repeats approx. every 4 seconds and will not stop unless you cut back power to descend. Its been difficult to not notice it now. I have uploaded a recording to YouTube.
The following changes in power are made to aid in identifying the sound through the engine sounds:- Power and prop full forward maintained soon after rotation
- Prop brought back to 2400 RPM
- Power brought back to 75%
Here is a screenshot of part of the spectrogram from the audio I captured of the Arrow III. The arrows point to each double or single sound of each sequence. Its in threes.
Here is a link to the audio sample. Sorry about the unnecessary video, took forever to upload and forgot to just upload an mp3: YouTube Arrow III Sound Sample
If anything else is needed, please advise.
Thank you
