VPI SDS producing hum at 45rpm

Noticed that my SDS is producing a hum when the needle is in the groove, but only when running the SDS at 45 rpm. If I lift the needle up, the hum stops. If I leave the SDS at 33rpm and change the belt to the 45rpm position, I can play LPs without a hum.

I just switched from a Dynavector 20SL to a Dynavector XV-1s today. My SDS is adjacent to my turntable. If the proximity were to be responsible, wouldn't I hear it all the time - not just when the needle comes in contact with the LP?

Thanks in advance for any advice.