I have a 250 IS and mine also has this squeeking noise. Yes, mine too is when I'm on a bumpy road. It seems to only happen if it's sunny or warm outside, never in the morning. I had them look at it once and they said they found a rattle from the rear sub frame and repaired something to do with that but it did not fix the problem. They offered to let me come in anytime it's making the noise and they'll look at it but I live an hour and a half away from the dealership so it's hard for me to "drop by." They've been very nice about the whole thing, it's just been really frustrating. What an annoying noise!
