Dear Adam, I haven't a clue. Typical reasons for what you are hearing are that some cartridges can pick up EMI from some turntable motors. Thus, as the cartridge nears the center of the LP, it gets closer and closer to the motor until at some point, hum becomes audible. But this is "hum", not "rumble". Grado and Decca cartridges, IIRC, used to be culprits in this sort of issue (the EMI-induced hum, I mean). Rumble is hard to describe, except I think of it as low-pitched and irregular in frequency. Like Halcro and some others suggested, make sure that there is no mechanical contact being made between arm wand and chassis or platter or other parts of the tonearm, as it sweeps across its arc. Does the cartidge get all the way to the run-out grooves, or does it get hung up? If the latter, then it is some sort of mechanical problem.
