Vibration.....Driveshaft?

Were you a flier in the Air Force? I never got that far, but had my civilian CFII and Twin Instructor

No, I wasn't a pilot in the USAF - just a civil engineer. I did get my multi-engine instrument rating though. I bought a gyrocopter years ago (just like the one in the original Mad Max) and it's on my To Do list to get it flying in the future. Till then I just fly my buddy's Archer. I'll bet that since you are an instructor, you probably fly a lot of different planes. Is that what you do for a living?