Eugene,
My '93 es300 had a loud rattle at certain rev range. It came from the air conditioner compressor, or the pipes from it. Holding a pipe firmly deadened the noise. I used a couple of nylon cable ties to pull the 2 pipes together a bit. This almost, but not quite, fixed the problem. The air conditioner works fine, by the way. Maybe its some serious internal problem, but I don't think so.