Checked the diff oil and it's full, can't say it smelt of cat pee though, just like oil. Whether it's the right stuff I don't know, had the car since March and it's not been changed since.
Did have the rear tracking adjusted (or so they told me, I watched them do it and all I saw was them struggling to loosen a nut and then seem to give up and tell me they'd done it) and I've only really noticed the noise since then. Everything underneath seems tight. I thought about a geometry check at Lexus but at £111.00 not sure whether it would solve anything.
Thanks for your responses.
My thought's are they've overdone the tracking causing the tyres to "scrub", hence the increased road noise :winky: