PA-28-140D Fluctuating RPM indicator

I purchased a 1971 Cherokee 140 shortly after getting back into flying the
first of this year. It is a marvelous plane, relatively inexpensive to fly
and to maintain.

The other night while flying with my instructor he asked me if I typically
got this much vibration. I told him that it felt normal, however when I
looked at the GPS on the steering column it was vibrating so much I couldn't
read it. All indicators were normal, the engine sounded fine, the one
exception was the RPM indicator was moving back and forth a couple of
hundred RPM from the 2500 I had it set at.

I haven't had a chance to fly again and my mechanic is on vacation until
tomorrow.

Any thoughts or ideas?

Charles Graef