I had this on my RX400h. It was the actuator that moved the airflow from windscreen vents to floor. There was something in the mechanism (a lever I think) that failed and then it kept clicking away trying to move the lever up and down.
The actuator was accessible under the steering column and behind the foot brake. I had to dismantle the foot brake, move it out the way, remove some trim and then reach up to remove 3 screws that held the actuator in place. An absolute pig to do, using a screwdriver ratchet (Philips head) and I had to bend myself into a pretzel to do it. I put in a second hand actuator - this was 7 years ago and it's still fine. It was a choice of 2nd hand actuator £25 + my time, vs £000s getting an independent to do it.