I had one for a long time on a 4runner I don't know how many times I had it checked each time it went in for service or anytime it went up on a lift we could never find it, then one day I bumped into the front bumper apparently at just the right angle and it made the noise, turned out is was a loose mounting bolt for the bumper and it only wiggled one way otherwise it would appear tight. another one on a silverado was the heat shield around the motor it was bent just enough that it would touch on the exhaust manifold at a certain point took a while to find that one also just happened to notice it one day while looking at something else. things like that will drive you nuts and make you think there is something else wrong