My buddy Mike had the EXACT same vibration in his excavator cab last year. He checked all the mounting bolts first, like you said, but they were already tight. Then his mechanic found a small crack in the frame right under the cab seat. They had to weld it up, which took a whole afternoon, but the vibration COMPLETELY went away. Mike said it felt like driving a new machine after that. It just shows you gotta look beyond the obvious sometimes.

Agree, our brains love to make mysteries out of nothing. My shampoo bottle lasted months longer once, and I finally figured out I was washing my hair less often. It's just small changes in habit that we don't notice right away.