I take it this has developed and wasn't there from first installation.
My initial response would be to take the motor out and check if anything has worked loose or broken off.
If it is all still tight, clutch, flywheel, adaptor, I would put a dial test gauge on it to check for runout and to make sure it is still running true.
If it is I would then check the fan blades are all still there and that any balancing weights if any are still there.
Then arrange for it to be dynamically balanced, at full speed, and with each component added on as the balancing is done.
The balancing is important, especially due to the high speeds.
make sure the balancing shop spins the armature up to your max running speed and balances it first. Then add the coupler and balance, then the flywheel, clutch cover etc, balancing as each part is added.
With a mark to align each component, it can then be reassembled in the motor frame in the same configuration before refitting in the car.
Oh, just to add.
If you don't have a tach then you can work out the gear ratios and tyre rolling circumference and determine the motor speed based on road speed in one of the gears.
Also if your car has a speedo and a tach (not working) in the original instrument pod then often using 4th gear the speedo needle points to the same position as the tach needle would. So if you drove so that the speedo needle pointed straight up in 4th then the motor speed would be roughly where the tach needle would point to straight upwards too.
