Hi Guys:
Thanks for all your help, here is the latest update:
Got my car booked in yesterday at manchester lexus, showed them the noise on arrival, and the problem was confirmed: the noise and the funny sound after switch off the engine are both cased by the "vaccum switch valve", ( thanks to Boris Vassilieff's hint).
They sorted the car out today under warranty and provided me with 2 courtesy cars:(driving both at the sme time? No, no, no).
The first one was their manager's ride,( an IS 200d) and I had it for half a day then was told need to drop the car back because there were some personal belongs left in the boot, I then been given an IS 250 Auto, for the rest of the day and the evening.
P.S, the IS250 drives like a dream, comfortable, quiet, got the power whenever needed. the 200d was just like the green version of the 220d, not much different.
Ok then, in one word, great! Theyve done a nice job, credit to lexus manchester!
P.S again:
After the vaccum switch valve was replaced, the car feels very smooth when accelerating from 1400rpm, and stronger after 1800rpm.
before, it was like hesitating under 1800rpm, then strong pulling power after 2200rpm.
Ok, thanks guys and good luck!