I had a similar problem with my two-year old IS. It was wheel bearings on the LHS. It was only noticable on roundabouts, when the bearings were loaded, any sharper turn than a roundabout meant I was going slower so it wasn't noticable. Lexus Cardiff repalced the bearings under warranty at the 50k mile service and no problems since! Incidentally this is the only problem I've had with the car since I picked it up with 18k miles on it. I love it.