2000 GMC Sierra POPING AND THUMPING

At that mileage I would inspect all bushings to be sure they are still intact. Check sway bar end links and center bushings, shock bushings as well. It is something solid if you can feel it in the floor. Also lift and support front end and check for play in the wheels, to be sure the wheel bearings are good (noise turning right). Generaly it just needs a good front end inspection to see whats wrong, if it is that bad, hopefully it is obvious