Is it also possible that the muffler could have bounced or been bumped into the drive shaft?
Haven't seen harmonics mentioned so I'll throw it out there as a possible contributing factor. If you are racing around off road it's possible you hit some weird frequency between the front and rear bouncing around at different rates and moving around relative to each other. If it's a track, that might be repeated each lap.
More likely heat or muffler related though.
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