Sounds like it could be a u-joint. Get under it and give them a wiggle. My rear end is so worn that it pops some when going from Park to Reverse. Yours could be the same way. Hard to say.
When you say it is RPM related, do you mean speed related instead? If you go the same speed in a different gear (a change in RPM but a constant speed), does it sound the same? If not, I would think that would rule out the rear end.
Good luck,
Slonaker
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