I had bald tires on the back on my f150. Got 2 brand new tires and now when i slow down or turn i hear a thumping noise!

 

What type of tires did you install? Are the lug nuts all tightened? Sounds silly, but maybe the tire shop forgot to tighten then.

 

Thumping noise from a tire is usually a broken belt. Any vibrations? shimmers? rear is harder to feel these, but they would still be noticeable. Take it back to your tire dealer and have them check the tires. ( I used to work at Good Year back in the day) New tires often are out of round as well, this will also cause thumping. And like Bucko said. Loose lugs. If you went with an MT tire ( Mud Terrain) then the out of round issues increase with the aggressive tread. I remember putting 10 tires on one truck with the MT tires on it for this reason.