Hi All,

It sounds like it could be the arm/cartridge combination is producing a low reasonces frequency. This means it will produce a greater output when it gets near a certain frequency. The ideal reasonance frequency is around 10Hz ish. But if much lower, you will get cone flap. (the reproduction of warps). Also you  could get an increase in rumble from the turntable.

The turntables will be picking up various vibrations and this could be adding to the effect. Also there will be frequencies on the record that will cause the cones to flap.

Try switching your deck on without a record on the platter. Turn the volume up, can you hear any rumble or see any cone flap? Turn the volume down, put a record on. Switch the turntable off & put the stylus on the record. Don't turn the on but slowing turn up the volume. Do you hear any noise (howling etc)? Do you see any cone flapping. Also play a intro or outro groove on a record. The parts before or after the recorded groove. Look at the cone, is it flapping?

As a rule, this is not too much of a problem but better turntables the less this effect is.

SCIDB