Problems with SME 10 Turntable

Before I call Sumiko for advice, I'd like to run a problem by you folks. On the lead in groove [and for the first minute of music], I seem to be getting a slightly audible sub-harmonic rumble. I'm using monitors that go down to 35 Hz, and I can actually watch the woofer pumping, and the cartridge bounce at a certain point on the outer grooves. It is better on some records, especially audiophile pressings.

The other day, I slightly moved my TT stand, which is on casters, and sits on carpet. The stand is also rather wobbly, as it's not firmly anchored with spikes. But the TT sits on a Symposium Super platform. I tried to play a record that was still clamped to the table, and had played perfectly the day before. The rumble was so bad that I couldn't play the record! The stylus actually bounced out of the grooves.

The SME is about 6 years old, and only used occasionally. So my question is...can this be some type of oscillation from the inadequate stand [the SME 10 is an unsuspended table design], or possible belt wear or a flat spot on the belt. I haven't fired up the SME again, as there have been thunderstorms lately, and I'm waiting for the stand's casters to sink back into the carpet.

Thanks to all in advance! And I know that my stand will have to be replaced, but funds are somewhat tight right now.