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Rx400H Rough Engine/transmission

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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.

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Jacking the car seems to be obvious in that case..

Putting car in 2wd service mode or 4wd service mode could be required (check service manual how to do so without lexus diagnostic software) this mode allow you working the engine all the time (not switching to battery)

When jacked try to accelerate to get required speed. (deactivating esp could be required too- not sure about it in audi i had to switch off abs cables from each wheel otherwies abs /esp will not rotate wheel whithout ground uder it)

You could also check (as a first) optcaly the brake pads and discs- when stuck smth juddering on the car is typical (not only when braking)..

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