Al Gilson wrote:

We have a 1964 Cessna 172 E with the 145hp Continental 0-300D and a
McCauley fixed pitch prop. Last Saturday, I flew the aircraft for about
an hour. We were at about 3,500 msl with an air temp of about 42F.
There were some low clouds at about 4,200 msl. No problems. Later in
the day, my plane partner took up the aircraft for his biennial flight
review. They also flew around 3,500 msl with the same air temp. A
light drizzle was now falling in places.

They noticed a strange vibration in the aircraft between about 1,700 to
2,200 rpm. They described it as a "pulsing" vibration and likened it to
the vibration/sound made in a twin when the engines are slightly out of
snyc. After the usual checks for carb ice, mixture control, etc., one
speculation was that one of the rubber motor mounts was worn and
transmitting the vibration to the frame.

Needless to say, the flight was shortened.

Have the same plane, no symptoms yet... However our previous C152 had
something like that, only during takeoff turn to downwind and everything
was spooling and speeding up. After replacement with Lord engine mounts,
problem went away...

Look at the cowling to spinner angle, if the engine is sagging in the
mounts, you should be able to see it there...