There should be no clunk when switching from EV mode to ICE and back to EV. But, I doubt it is the transmission. It's more likely engine / transmission mounts or something in the front end suspension / cv joints / maybe steering. What are the inputs to the car when it happens? accelerating, decelerating, braking, uphill, downhill, coasting, cruising, low speed, high speed and so forth? How many miles on car? It sounds perhaps that when there is a change in torque requirements (whether in EV or ICE), you sometimes get a clunk (like one does when a CV joint has failed). Have you had the car serviced recently? If so what did they do?