Vibration that comes and goes

[AKADriver]

Just starting to characterize this after getting my car back together back in the spring and driving it more. Under particular conditions, I would say when the engine is above 'lugging' RPM but still below the powerband and I tip into the throttle, I get a lot of vibration through the floor. Noticeable when, say, I'm cruising at 55mph and going uphill.

It's a high frequency vibration that seems to follow engine RPM, not the low-frequency vibration of an unbalanced wheel or bad axle or suspension issue. I've tried downshifting to see if it shifts the frequency upward but that just tends to make it subside.

It's most noticeable in the floor/seat. Not much in the shifter, nothing through the steering.

My first thought is that my header or exhaust are contacting the floorpan when the engine tips over towards the right under load. I'm using OEM-spec mounts so I expect some movement. But that would be a lot. But that's the first thing I'm going to check, I guess if there are fresh witness marks on the floorpan or cat heat shield. Header is an ISR 4-2-1, is hitting the floor a known issue with those? In my Miata days a lot of header brands were known for being either too close to the tunnel or too close to the bellhousing and sometimes needed a precision dent.

It's not an engine, harmonic balancer, or flywheel problem, I don't think, because I can rev it out in neutral and feel nothing.

Second thought is a driveshaft problem, but I don't get any clunking and there wasn't any unusual play in the u-joints and the center support bearing turns freely and quietly. The driveshaft is OEM. But it's in the right frequency range to be a driveshaft issue.

I actually wish I had a transducer and oscilloscope so I could record the vibration and match it to engine rpm or driveshaft rpm. Much easier to track it down