Can anyone shed light on an strange noise that emanates from my engine
on startup? The engine is a basic 912UL, with about 520 hours on it.
The noise is an electronic note, almost flutelike, around G in pitch.
After startup, and idle at about 2200 RPM, you hear this noise through
the firewall, loud enough for a passenger to comment. With increasing
RPM, it doesn't increase much in pitch, just slightly, and therefore I
think it must somehow be related to the alternator output, or possible
the voltage regulator (Ducati); and not related to any noisy bearings in
anything.
You cannot detect this noise if you stand close to the running engine,
with the cowling off. It seems that the firewall acts as a resonating
surface to amplify it to inside the cockpit.
The noise goes for about 5 minutes during warm-up, then quite suddenly
it completely disappears, usually at about the time of taxying out. All
through this startup, the engine runs smoothly, and temperatures and
pressures behave normally. The big spike-protecting capacitor charges
up, and after a minute or two from startup, the voltmeter shows a switch
---From 12 volt (not charging) to 14 volt (charging), All quite normal,
and seemingly unrelated to the noise.
This noise didn't happen when the engine was new. It began after about
300 hours of engine time. It is particularly noticeable after sometime
without engine operation, say 2 weeks or longer.
Any ideas on this mystery noise gratefully received!
Richard Lamprey
Europa Classic, 5Y-LRY
Nairobi, Kenya
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