I have been searching around but haven't found a right answer. It happens to my 96 LS400. Every morning, even in summer, when I start up the engine, drive and accelerate up to about 1700 rpm and I can hear the knocking sound from the engine (somewhere behind the air vent) the the power was not great. My AC and fans are off. The noise continues for about less than 10 minutes and gone after that, and the car power is back. Sometimes I let it warm up for 10 minutes, but when I drive, the noise is still there for a few minutes. I use Shell and Exxon premium gas. Sparks plugs and cables , pcv, and ECT sensor are new. Am I missing anything?