Intermittent vibration issue

Hi all,

Wondering if any of you have had this issue described below up and any ideas of what's causing it. I drive a 2012 Prius V with 170K kms/105K miles. Kept up with the egr cleaning, fluid changes (oil, transmission, coolant), I need to have the spark plugs changed at the next service.

I've had an intermittent vibration that feels like an unbalanced or flat tire that's variable with speed but the engine is running smooth when this happens and doesn't seem to be tied with the ICE RPM. Braking doesn't change the vibration and it's not coming through the steering wheel. It happens very infrequently, about once a month max and disappears within 5-10 mins of starting, i.e. I'll feel it, look for a safe place to pull over and it's gone. It seems to happen when I'm cruising and not taking off from a stop. Car drive perfectly otherwise

I've had the tires rebalanced and rotated twice (Costco standard the first time, road force balanced second time), I've looked for play in the wheel hubs, visually inspected the CV axles and the suspension to see if anything looks wrong but haven't seen it.

I'm going to ask my mechanic to try and diagnose it thinking it might be difficult since it happens so infrequently

 

It seems to start when I’m at around 50 to 70 kph/30 to 40 mph but is variable with speed. Both going straight or turning.