Road noise before and after new tires.

Got a 2005 Subaru Legacy Wagon 2.5i. I noticed a while back when driving a dull humming coming from what seems like the back/mid of the car. It almost sounds like I am in the wrong gear and the engine is revving (not as loud but similar) but the tac indicate all is fine, and car feels fine when driving. Thinking it was just weird road noise, I got new tries put on and balanced, but still it persists. Any idea what it might be?