Vibration at any speed.

[RA1]

So I bought a 2001 986 Boxster a few months back and I have been dealing with a vibration at all speeds. The sound is very close to a "whoosh" with every rotation of the tire (gets faster and faster with speed). It is pretty noticeable with the top up, not very noticeable with the top down. The whoosh is coming from the rear as their are no vibrations in the steering wheel but a slight vibration in my seat. At highway speeds it sounds a lot like someone driving with offroad tires. The car has 106k on the clock. When I first bought the car, both me and the mechanic that did the PPI failed to notice that the dates and the tread on the rear tires didn't match. Apparently the previous owner got a flat on the tire just 1k miles before I bought the car and he replaced just the one tire. I immediately replaced both rear tires and balanced them. The new tires seemingly made the noise worse, so I got them re-balanced and sure enough, the sound continues. The looming thought in my head is that driving the car on the two mismatched tires messed up the differential pretty bad... What am I missing? Besides this one issue the car is in surprisingly fantastic shape. Almost perfect cosmetically and the PPI revealed really great mechanically. No other issues besides this (knock on wood) and the car has been an absolute dream to own.