I have a 99' ES 5spd with 102K. At cold start up I notice a loud knock/bang. It only happens at start-up when the car has been sitting for about 8 hours. It happens about 80 percent of the time and seems to sound like it is coming from the belt side of the engine. Does anyone have any idea what this might be. I don't think that it is the cold start-up butterfly thinmg in the intake.

 

It sounds like a hammer hits the timing belt cover with a medium force swing as soon as the engine fires. It only occurs once at long-sitting cold-start up