I was cleaning all the sinks (some were starting to drain really slow because of hair in them). While in one of the bathrooms, I decided to put the shop shop-vac I was using to the drain hole of a tub and sure enough it started to make a repeated thump sound every couple of seconds. Any ideas on what this can be?

Backed up water refilling the p trap? Partially plugged vent pipe with water in it? There is nothing mechanical in a drain system to make that noise.

Except for the plunger on the waste and overflow.