At exactly 2,350 rpm, the pulsing vibration occured and I was able to
experience it. It was about 1 cycle per second and did sound/feel like an
out of sync twin. It disappeared at 2,300 or 2,400 rpm. Other than that,
the engine ran as always.

How many hours on the engine? How old are the engine mounts?

As the rubber mounts age, the engine sags a bit, and they become stiffer,
transferring more vibration to the airframe. You may have just developed a
resonant at that frequency range? And perhaps the unusual atmospheric
conditions made it more apparent?

Or, it could be something more serious in the offing.

Ain't airplane ownership fun?

;-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"