Matt,

If that noise is something that just started, it sounds like something is loose, or broke.

I had an R/C airplane do that, and couldn't figure where the noise was coming from. Finally, pulled the engine out, and found one end of the aluminum engine mount broke loose, so the rest of the mount on that side was vibrating against the wood structure.

Bill

Everything is tight.  I am running an extension shaft on my OS .35S, so could it be that, that's vibrating.

The extension itself should not be causing any vibration but.... the prop balance becomes more critical as the distance from the end of the engine case is increased. It amplifies any vibration. I'd double check the prop balance.

Hi Matt.
I have been around I-Beams since the mid fifties and the sound is typical of that wing construction. Back then most were Fox powered and there was vibration no matter what was right or wrong. The wing is basically a drum with all of its open structure and tight covering top and bottom. Even the I-Beam combat planes of the day (similar to a Ringmaster) had that sound and one always knew when an I-Beam was flying. Make sure everything is tight and prop balanced, try it without the spinner and even slightly change the size and brand of the prop and launch RPM. If the sound persists then it is what it is or Grace cast a spell on it!!!!

Enjoy,

Don Boka.

Maybe try a prop that will put less load on the engine, it could be predetonating.
Don

Hi Matt,
If you don't have any installed, you may try thin discs of sandpaper between the prop driver on the engine and extension shaft, and between the extension and spinner backplate, too. Real thin 1200 or so might do it. Balance the prop and spinner as a unit and mark them with a dot of paint so you orient them the same way every time. It might help.
A friend had a Formula 1 racing airplane (one you race with yourself inside of it) and he changed props and it vibrated very badly at racing rpm. He re-clocked the prop 180 degrees on the crankshaft and the vibration went away. Those are 200 cubic inch four cylinder engines, but similar effects are known on our little 1 cylinder jobs. Changing some things might minimize the vibration you are having.
Chris...

Matt,  the change in fuel has probably made a change in rpm.  If you check the rpm with the 5% and then set rpm the same with 10% you will get your echo chamber back.  DOC Holliday

I had this problem with a Fox 35 powered Gieseke Nobler a few years ago.  Ran Superfuel and a Top Flite 10x6 Super M prop. With the first test flights, I thought it worked great, until it would hit resonance every now and then in level flight.  It would rattle so hard I thought it might disintegrate, then, the fuel delivery to the engine would be disturbed, the rpm would change, the resonance would stop, the engine would return to its normal rpm, and the cycle would (might) repeat.  After two or three flights with this alarming issue, I "hid" from the problem by using a 10x5 Super M and running some more rpm.  The problem never came back, but the airplane suffered a major structural failure only 100 flights later, something I attribute to the vibration it was exposed to.  Resonance can be very destructive...