Help me figure out a noise....

[Brantel]

So a few weeks ago I was on a nice smooth solo flight and I started noticing a noise that I am not used to hearing.

The noise sounds sort of like a high pitched whine similar to alternator noise.

First I have ruled out the alternator (electrical noise, not bearings). I can hear it without the headsets on. I have also turned off the alternator in flight and the noise remains.

On several flights after first noticing it, I noticed that the volume of this noise appeared to change on different flights. Weird....

So on my last flight I accidently discovered that the noise volume is directly related to how much I open the cabin heat flapper thingy.

If it is closed, the noise almost goes completely away.

If I open it, it increases in volume up to 100% volume when the gets to about 50% open.

If I open the door more than 50% it starts to reduce the volume of the noise.

One theory I have is that it is some sort of air flow noise that is causing something to resonate and the different volume is due to the changes in airflow thru my heat system. (standard Van's with a Robbin's Wings heat muff and a standard stainless heat valve that dumps to the lower cowl when closed)

Another theory is that the angle of the heat valve door is reflecting the noise to my ears when at the proper angle making it sound louder at a certain spot.

Any ideas on what the source of the noise could be?

For context: I have a mechanical fuel pump, 2 Pmags, and a PCU5000X prop governor on my accessory case and that is it. The engine is a bone stock narrow deck carbed O-360.

[Mike S]

Does it make the noise on the ground?

Alternator bearing going bad?

Prop gov bearing going bad?

Small furry animal crawled up under the hood?

[Brantel]

Originally Posted by Mike S

Does it make the noise on the ground?

Alternator bearing going bad?

Prop gov bearing going bad?

Small furry animal crawled up under the hood?

On the ground I have not heard it but I can’t get the rpm up to my normal without anchoring the plane to something.

I have looked around but have not seen any obvious issues.

[Tracer 10]

Check your Aeroducting at each connection that is secured with a hose clamp. Disconnect each clamp & slide the Aeroduct off and insure there is it a piece of it protruding into the air pathway.

[Ralph Inkster]

I once had a fibreglass windshield fairing come loose in flight, it started resonating in a high pitched screech. Maybe you are experiencing something similar that reacts to varying cabin pressures as you open & close the heater valve.

[lr172]

Run some tests in flight to see if noise changes in any way with changes in RPM. That can help to eliminate many of the engine produced noises.

I would also look for a bird, hornet, mice nest somewhere in the scat tubing for the heater circuit

Larry

[skylor]

Originally Posted by lr172

Run some tests in flight to see if noise changes in any way with changes in RPM. That can help to eliminate many of the engine produced noises.

I would also look for a bird, hornet, mice nest somewhere in the scat tubing for the heater circuit

Larry

Also try to see if the noise changes in pitch or volume at different air speeds. This should help identify if the noise is coming from something vibrating from wind or slipstream.

Skylor

[N661DJ]

I had a similar experience with my 8, frequency of the noise changed with airspeed, not with engine RPM.
I took a friend with me and when the noise started he proceeded to touch and push on various areas of the airframe and canopy. Finally when he touched the lower rear portion, left side, of the canopy skirt where it contacts the side of the fuselage, the noise stopped. A small piece of felt weather stripping attached to the inside of the canopy skirt cured the problem.
Dick

[jdiehl]

A few years ago I had a random whining noise that was quite noticeable in flight. Bottom line, the short tachometer cable needed lube. Problem solved.

[Brantel]

Originally Posted by jdiehl

A few years ago I had a random whining noise that was quite noticeable in flight. Bottom line, the short tachometer cable needed lube. Problem solved.

I forgot to mention that I don’t have a tach cable. My tach drive is covered with a cap.