I have a VPI TNT 4, a Wheaton Triplanar 6 tonearm, and a My Sonic Lab Signature Gold cartridge as well as an VPI SDS. I had noticed on occasion a slight, I'll call it hum, when running speed at 45 rpm (with a Transfiguration Phoenix cartridge), but now that I've had the MSL, it is much more noticeable. I not only hear it between tracks, but also during tracks. It seems to be a medium frequency, not your typical hum, but higher freqeency than that. It seems somehow in line witih the turning of the record, just looking at it whlie listening (if that makes any sense). I haven't nogtised anything at 33.3 rpm.
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Is your motor housing touching your plinth? The plug on my motor for my Scoutmaster was touching the plinth and causing a constant hum. I found it when I switched to 45 rpm and the hum increased in pitch. Drove me nuts before I figured it out.
If there's no hum when the cartridge isn't on the LP, then in my experience this is usually motor vibration transmitted via the belt.
JohnK