Plane - Hangar 9 Advance 40
Engine - OS 46ax, stock muffler
Prop - APC 10/7
Fuel - Cool Power 10%

This plane flies WONDERFULLY! The engine, as old and used as it is, fires up with just a tap from the starter, even after sitting for a week. Lands smooth as glass.

HERE'S the weird thing:

When I start the engine, she fires right up. Cold, hot weaher, dry, humid, doesn't matter. I'll rev it up to full, smooth as can be, great transistion. Hold it nose up, maintains, no sag. HOWEVER, when it's in the air, there's a weird
gurgling/vibration sound at certain RPMS. I know, it must be flutter, right? Couldn't be, cuz I checked all my surfaces, no give. Well, just to make sure, I took her way way up, yanked the nose down, she's diving straight down, fast as hell. Ipull up kinda sharp like, she levels like she's on rails. No flutter, not even a hint of it. So I bring her down, check the canopy, no give. All my surfaces are still tight. Pushrods don't flex, They're reinforced. I check the engine. Crankshaft just as solid as new, no roughness in turning it. Head is tight. Prop is very secure, and perfectly balanced. Even tried different props, same thing. Basially, there's nothing loose on the plane. And again, the only time it makes this sound is when it's flying, and a little past half stick up to full (throttle).

It acually sounds kinda cool, like the engine's got more balls than it actually does, but I'm concerned because it doesn't seem like a sound this engine should make, and I've never heard any others do it.

Is there anything I might be missing in my inpections? Like Isaid, the plane flies great, but if there is a problem that might get worse down the line, Ireally don't want to lose the plane because of negligence, or worse, hurt someone.

Any ideas anyone?