Engine buzzy at low MP but only in flight.

[Shadrach]

I am not sure when this started. Perhaps I have just become more sensitive because I am flying multiple aircraft types and engines.

I feel a harmonic through the airframe when at low MP in the pattern. (prop driving engine).  This harmonic does not exist in any other operational regime. Engine is smooth at all RPMS and MPs except for descending power off.  The non-counterbalanced, IO-360-A1A does have a restricted RPM range (1950-2350) and I have always been able to feel a slight "buzz" in that range when transitioning through the caution range.  However, now it feels like the buzz doesn't completely go away after dropping below the caution range.  This buzz is not present on the ground. The engine is smooth throughout the RPM range during static runup.

I have given consideration to the following:

Intake leak - I have not ruled this out, but there are no other indications that an intake leak is present.

Ring Flutter - I have no experience with this phenomenon but have read that it can cause vibration at low power.

Propeller imbalance - Seems unlikely as propeller is smooth under most operating conditions.

Motor mount - A cursory inspection revealed nothing. My motor mount is powder coated in white. The finish is in great shape which should make cracks obvious. I will give it a more thorough look soon.

Idle - My idle is set very low, so low in fact, that the engine will quit on the ground if I pull the throttle all the way out.

All thoughts appreciated!

[takair]

Some things to consider:

Rubber mounts have hardened or sagging. This would change the position of the engine especially when changing power. Might be causing some slight interference. 
 

Exhaust hangers broken. 
 

Baffles. Especially with dog house, I’ve had cracked baffles drive me crazy with some noise I could not pin down. Would change depending on flight dynamics. Worse is a broken baffle which once scared me into diverting. It was minor but made an awful noise, like the engine was coming apart. 

[N201MKTurbo]

Cowling kissing engine, or something in the cowling resonating with the low power.