Noisy Outback

What’s my problem. I get a loud noise from the rear of my 2005 Subaru Outback, 67k miles.
(No it’s not my dog Pete)
Starts as a WHOP, WHOP, WHOP that speeds up as the car speeds up 'til it’s a loud drone at cruising speed. I thought it was older tires but changed those Saturday. No effect. What do you think?

Tires would have been my first suggestion, but you’ve already replaced them, so that’s not it.

You did replace all four, right? That’s important on a Subaru.

I suggest you have your mechanic drive the car and see what he/she says. Whatever it is, it shouldn’t be happening.