I don't own a lexus, just browsing the forums as i'm after a gs300 sport, but i recently had a squeeking noise on my Prelude 2.2 vtec, ruddy awful noise, it went away when braking, but came back when you took your foot off the brake, so i took the pads off, noticed brake dust sitting on the pads, so had a closer inspection, and the pad braking surface had come away from the backing plate, because it was moving against the disc quite rapidly, it made a squeeking noise.
These were a new set of pads on the front and 3 out of the 4 had come away on one side, so returned them to the shop and got a new set, been fine since, could be that i overcooked them, still not sure why it did it, and i'm not sure if this will apply to you, but the symptoms sounded similar. Good luck finding the problem, and i hope it doesn't cost to much to fix