Did post this in the UK forum but nobody else seems to have had the same problem. Between about 20-30mph, on light throttle the engine/transmission of my 2006 RX400h makes a low groaning sound, sending a vibration through to the cabin. The engine just seems to be very rough at this speeds. After exactly 30mph, everything is very smooth. It is also very smooth when running in EV mode at any speed. The harshness/vibration only comes on when accelerating, so have ruled out tyres etc. The mileage is 85,000, and it has had a recent major service at a Lexus dealer. I use premium fuel. The Lexus dealer had a look at the car for 1 hour and inspected the exhaust, engine mountings etc. but could find no faults. I went on a test drive with the service technician and he did notice the vibration. The dealer said I will need to let them have the car for a whole day for them to try and find the cause, but I'm a little hesitant about paying for this as there is no guarentee they will find the problem. Has anyone noticed a similar problem, or have any idea what the issue could be? Would be very grateful for any ideas.