I had an annoying buzz/rattle that would come and go.
It was a harmonic that only happened while going through 37mph, both up and down.
So I hired a neighbors daughter to find it. She was 8 years old and tiny for her age. Took about two hours for her to track it down. She crawled under the dash, under the seats, front and back. And even rode in the trunk. I found a long street in an industrial area that was shut down on the weekends and just drove back and forth going from 35 to 39 and back.
She had an orange crayon and I had her mark where she was able to stop the buzz by grabbing the part, or pushing on a panel. She found three places and I fixed the buzz by welding a poorly welded panel in the trunk, and spreading panels behind the dash apart with a plastic pry bar and putting thick pieces of flannel between them.
I paid her $40.00 and her mom told her she had found a career, since it was more than she was making.