My 08 600HL (159,000 miles) was making a terrible grinding noise when accelerating. I was freaking out thinking it was a fuel issue or even worse, an engine issue! There was no code so it was a mystery to me. I took it in and had it diagnosed at Lexus and found out it was the left front CV Axle! Apparently the boot had worn down and all the grease was leaking out, causing the grinding noise! Just wanted to post this just in case anyone else encounters this problem.