Are RPM needle fluctuations at altitude normal?

[jruhnke]

I live at sea level, and much of my flying is local and below 3000'. However, when I head out away from home, I like to climb a bit. I have noticed that above ~4000' or so, the RPM needle in my PA28-180 exhibits noticeable bobbing (50-250 RPM or so). The fluctuations increase with altitude.

These indications do not appear to be "real" engine readings...I can detect no change in engine tone, nor any corresponding effect on airspeed/groundspeed, etc. The needle movements are fairly quick, on the order of .5Hz or so.

I don't have much experience with different planes. Is this normal, or should I be troubleshooting something? What causes this behavior??

Thanks,
Jim

 

[PropFan]

 

[jruhnke]

 

[TEG916]

 

[DaveB]

 

[jruhnke]

 

[magman]

 

[dkallen]

 

[jruhnke]