Hi Alan,
Not sure if this helps but...I had an RX400H prior to my current model which developed an issue with a low level rumble which ONLY happened around take off/ accelerating that was linked, I believe, to the vehicle switching from ICE to battery mode.
Once the fault had developed a bit more, (dealer unable to find the fault on first two visits) I was able to recreate it on demand by accelerating and then dropping off the gas pedal slightly so the ICE stopped and battery mode kicked in, by controlling the throttle, I could continue the rumbling noise for as long as I wanted.
The dealer opened a case with Lexus UK but I never got a full solution as I traded up to my current model with them before I had a fix. Does that sound like your noise? Do you have a dashcam with voice recording capability that could ‘capture’ the noise to replay to the dealer?
