SC300 Rattling
 

My 1994 SC300 has118K miles and a very good maintenance record. It rattles at initialacceleration. Even at high rates of speed, I will have taken my foot off the accellorator and then accellorate and I hear the rattle. It otherwise runs great. I am using 94 octaine gas and a fuel additive (Jet fuel) and that seems to have some effect but does not eliminate it entirely. I took it to a local machanic who works on imports only and he can't find any loose linkage or other parts that would cause the sound. Is this inherant to the car?
thanks