As Jay asked:

There are about 500 SMOH on this engine and it was overhauled about 5 or
6 years ago. The engine mounts all look good with the exception of one
that has some very slight surface cracking.

Of course, we had plenty of ownership fun poking around in the engine
compartment and then raising our heart rates during our test flight.

What is interesting is that this all came on at once.

Still curious.

Al



Jay Honeck wrote:
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?

;-)