Hi,

Back when, I was the young man I used to be, I heard a story from a Ford dealers mechanic. I believe it was back in the mid 70's as I have not used a dealer since I bought my first and last Ford in '74.

There was a new car customer that kept complaining, while under under his or her 12k warranty, that at certain times, like when the car turned corner or ran over a pot hole or speed bump, there was a thump coming from the rear of the car.

The dealer ship drove the car numerous times and would occasionally hear the noise too.
They finally found out the noise was coming from a rear wheel well from an area of the back seat even though there was nothing there or loose upon every inspection they used.
They ended up cutting open a section body of panel, that was inside or outside the car, where it was determined that there could be a hidden "void" space. Inside the void they found a wrench, that had a note attached to it that read, "Had you looking didn't I?"

Phil