Measuring Turntable Platter Speed

I had a long-delayed second thought. The best course of action, IMO, would be to follow the procedures suggested by MC and oldhvy, and determine whether there is a change in the condition. There are also obvious things, like observing the platter in operation to determine whether it is rotating eccentrically or off axis, either of which would suggest a major problem with the bearing and/or spindle shaft.  I wouldn't be so obsessed with measuring what one can already hear to be a problem.

I know exactly how the RR works, Mijo. If the phenomenon that the OP is sensing lasts for less than one full rotation of the platter, I doubt he’d hear it, and the other devices mentioned would be even less likely to detect it.

Anyway, the RR is too expensive for this investigation.  No harm in trying the cheaper alternatives, but the ears tell the story here.

A strobe device may miss what you're trying to identify. The other devices mentioned would be even less likely to catch it, in my opinion. Best would be a Phoenix Engineering Roadrunner but that's too costly for this use. Your ears are free.