Troubleshooting Mystery Sound

[gammaxy]

My airplane has started making an unusual sound. It doesn't seem to be engine related since it doesn't change with rpm. It goes away at low speed and today I noticed it goes away completely with full left rudder. I've had it happen on a flight, then not noticed it for months, then noticed it again. The first time it happened, I thought it was a gear fairing vibrating against the fuselage floor, but I'm less convinced now. It seems to start after I've been airborne for a while.

https://www.youtube.com/watch?v=X1Y7Mu7G3Zk

Apologies for shooting a vertical video.

Any ideas? It's mildly annoying with my headset on, but it gets picked up by the microphone and amplified very well while I'm transmitting.

[SonexN76ET]

Hi Chris,

I can not quite make out the sound you are referring to in the video’s audio. Can you describe the sound that is troubling you?

I had a wheel pant that had loosened up a bit on the outboard axel side and that caused a noise that took me awhile to figure out. A relatively loose wire to the oil pressure sensor can cause a racket too as can loose baffle material. Loose stickers on the airframe can cause a noise as they flap and vibrate. I put some no skid stickers on my wing walk area and the glue on them did not adhere well to the aluminum and they made a buzzing and banging sound until the slipstream tore them off.

So, there are any number of small things that can cause unusual noises. I wish I could better pinpoint the sound you have experienced.

Jake

[gammaxy]

There's a high-frequency tone that goes away when I get below about 90mph in the video.

[Rynoth]

What I'm hearing sounds like a whistle, as in air passing a small oriface somewhere. Maybe even the gap beween wing and fuselage, canopy and fuselage, or some such.

[lutorm]

Did you forget to turn off the dive bomb siren? ;-P

[gammaxy]

I'm still getting the sound. I remembered removing the cover off the aft hole on the bottom fuselage skin, so I put it back on. I also flew without landing gear fairings to rule out that. None of this helped.

Here's a recording from today. The tone is lower in frequency and less pure than the last recording (840 hz with harmonics at 1680 and 2520hz). This is more like how it sounded the first time I noticed it.
https://www.youtube.com/watch?v=cToPagcG50o

[SonexN76ET]

Chris,

Your new video’s sound makes it much easier to hear the noise you are experiencing. Can you hear this on the ground? The video’s sound is reminiscent of the mechanical whine of a helicopter in flight. Can you hear any of this noise when you do a high powered run up on the ground? Could the prop hub be coming into contact with the cowling? Does the noise change with RPM or with airspeed? Any changes in oil pressure lately? If it is mechanical, I can only think of a few things it could be. Have you checked to make sure your secondary ignition and alternator components are secure? The reason I mention this is pumps can start to whine as they age indicating it is time for service and the alternator because some have reported the stator screws have loosened causing damage and or a loose trigger magnet shaft. Only other thing I can think of is the gears on the crank and camshaft somehow causing a whine. This is all pure speculation but might help you troubleshoot this noise.

Jake

[kevinh]

I had a similar sound pop up on my RV and it was very frustrating finding it (and it had been noise free for about a year prior). The approach that worked for me was slowly adding tape across everything that might whistle. The problem was eventually tracked down to a gap seal that had slightly shifted position. So I reinstalled the seal with a little bit of RTV to seal the small channel between the rubber and the aluminum.

[gammaxy]

Thanks guys.

I'm pretty sure it's not the engine or accessories since it doesn't seem to change intensity or frequency with power setting. I don't get it on the ground at any power setting and it always goes away at some point during the landing flare as I get below around 60mph. I suppose it could still be something vibrating due to airflow in the engine compartment. I did attempt to stop the propeller in flight to completely rule it out, but surprisingly, the propeller kept windmilling, but the sound didn't change. I admit though, it sounds exactly like I'd expect a broken generator to sound.

I do have tape on my wing gaps that I could probably remove to rule that out. A probably incorrect theory I thought of this morning is that the tailwheel is spinning in the wind and has a damaged race or bearing. I have tried applying my brakes in flight to rule out spinning main landing gear.

[Bill Sargent]

I cant actually hear the noise. Try holding on the brakes , I had issues early on with the cheap pressed wheel bearings on mine. The wheels rotate in flight and any bearing noise sounds really weird. This wont help if you are a nose dragger and it is the front wheel. Just a thought
Bill S
Sonex 160