Beezer

It sounds tinny and like two things are very close together. If you have the regular stereo, there is only a center mounted subwoofer in the package tray, the Levinson adds two small speakers on either side - either way not much chance they are making the sound. I wonder if the closing bars on the rear trunk lid are clanging. Could have been bent if you jammed something in the trunk like golf clubs. you can take the back seat out by pulling up on the front of the lower cushion, remove the bottom seat, then take the visible lower bolts off, lift up and remove the seat back. Takes only a few minutes but you need to get to the bottom of that noise.

I bought the 08 with premium and the stereo made me have buyers remorse. I hated it. I found a post regarding upgrading to the ML system and did it. The ML only comes on the NAV models which I don't have. You need to find a junkyard es350 with ML and get the amp (in the trunk) and 11 speakers with grills out of it. If you try to buy the parts new from Lexus it will run about $5,000+ not including the wires. The head unit (the part in the dash) is the same and all functionality remains perfect. It takes many hours and some wire splicing and added runs but the result is sooo much better. I agree that the ML is not the best system but it is very adequate and the standard radio is not. The standard speakers are plastic Pioneer with small magnets - the ML are paper cone and heavier magnet Harman Becker speakers. You may be able to upgrade the amp and speakers while keeping the factory headunit but I don't know.

From your list, the O2 sensor is the most likely source. They go due to the high heat / cooling cycles. In a distant second in line would be the coolant temp sensor followed by possible clogged gas filter (rare). My GS400 has about 80k but nothing has gone wrong yet. I'm no expert, just been around this for a while. If it was my car, I would not do anything for a few tanks of gas and see if the problem goes away. Then replace the O2 sensors.

Gas cap tight?

I'm a little new to this forum. Has anyone discussed how the built in phone is wired in a 99 gs400? I would like to hook my phone into that wiring. Schematic anyone? Thanks