2000 Pontiac Montana tire sounds like its flat

Lift the vehicle and gently place your hand over the tire and spin it. See if you feel any irregularities in the tread whatsoever. While it's up, shake and squeeze the steering and suspension components and check for looseness. Squeeze the tie rod end. Grasp the tire at top and bottom and rock it, or try to rock it. The only movement should be from sidewall flex. Grasp the tire at the sides and push and pull. Again, you should only get sidewall flex.

Check the shocks for leakage also. And perform what's known as a jounce test. Push down on the bumper a far as it will go with either your knee or foot and let it go. See how many times it bounces.