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I have a 2008 IS-250. My front left suspension area squeaks when air temp is low and the vehicle is going over bumps. Adding air pressure to the front left tire seems to reduce squeaking, as does increasing air temp. So my guess is the lower air pressure is changing the way weight is distributed through the suspension, which is causing the squeaking. It only occurs on the front left.
Additionally, front left brake is groaning/grinding while doing light braking at extremely low speeds (such as rolling with brake applied). Braking while turning hard (such as pulling into or backing out of a straight parking spot) creates the groaning noise as well as a cracking noise. When releasing from a full stop, there is a noticeable but quiet cracking/popping noise. Like the squeaking, this only occurs for the front left and seems to be affected by the air pressure in the tire.
Check your brake pads. Beware, like in my case the inner pad wore down more quickly that the outer pad...I had similar scratching sounds at low speeds, but mainly when turning left.