I've had this clicking noise coming from the underside of my car only at speeds of 15-25 mph with the loudest at 20 mph. I took the car to my mechanic a couple days ago and he tightened down everything in the engine bay to fix it but it was to no avail. Anyone have any idea what it could be? possibly a loose exhaust component?

 

Heat shield for your cats?

 

If it sounds metal on metal, could be the brakes. Our Sequoia does that at 5 MPH and under. Drive shaft has a slight chance of being loose caused by a rubber bushing that deteriorated...
Is it in the rear or front underside?