What would cause a grinding noise after a tire is replaced and the brakes are not worn?
What kind of noise and is it just one tire?
CV Joints? Rocks in the tire tread?
1999 Mercury front and humming noises can because by several things. Tire noises are the most common cause. A tire that is not wearing properly can cause a humming noise.
low gas, flat tire, broken gas
When they were repairing it they probably pushed a plug through your tire accidentally and left it, then put in another plug correctly to seal your tire.
Nasty kid next door tied his skateboard under there? Maybe tied his little brother? Loose suspension components, loose drive train parts, esp CV and universal joints, dangling muffler, dangling spare tire....
Your tire may be making a squeaking noise due to low tire pressure, worn-out brake pads, misaligned wheels, or a problem with the suspension system. It is important to have your tire inspected by a professional to determine the exact cause of the noise.
The break pad.
nail in tire, or rock
Bad tire? Try rotating and see if "thump" also moves
Check the bearings.