Erratic Motor Behavior Audio-Technica AT-LP120-USB

EDIT: For a discussion of this problem, see http://www.audiokarma.org/forums/show...

It's been established that "cogging" is normal for a turntable. However, audible knocking through speakers is not normal, which is why I was wondering about this motor behavior.

OLD DESCRIPTION:
So this strikes me as not a normal thing for a turntable motor to do, right? I just got this via UPS today from Amazon. With the platter on, it seems to spin at the right speed, as indicated by the strobe, but when you actually play a record, a knocking from the motor is audible. I've never had a turntable with a removable platter before, but is this normal? Has anyone else experienced this problem?

I put the nut on to exaggerate the effect. You can't see it but I switch from 33 to 45 rpm and back again. Same thing at both speeds.