Low Frequency Rumble from TT between songs

I'm sure it's in the songs too but I'm getting low frequency rumble now that I've switched from Harbeth 30.1s to some large base reflex studio monitors. Between songs and on the lead out groove I'm getting a rumble at mid to high volume. Is that an isolation issue or something else. Table on a wall shelf, about six feet from the BACK of the left speaker, so I'm not understanding why this would happen.