My RX450h has started making a similar sound, just the last couple of days. Only noticeable at slow speed, manoeuvring onto the driveway, parking etc..
My initial thoughts are a dry rubber bush or gaiter. I know the Merc I had sometimes suffered from this and it was the sound insulating bushing as the steering column passes through the bulkhead.
The car is due a service soon and I'll get the dealer to sort it as it's not that irritating and I'm sure they have come across it many times before and will know exactly what to apply and where.