This has been driving me crazy since i brought my car. On start-up my car rattles and its loud enough to hear from inside the car its not too loud. I think its coming from the top of the engine valve cover it happens when it sits for hours, i tired to show my instructor but it didnt do it when i started it up if anybody has had this problem before could help me. Also its a 1995 SC300 with 175000 on it.

 

does the rpms go above 2000 rpm when this happens? does your car idle a little bit on the high side when its warming up? if it does then you need to make sure base timing is set properly along with tps and working IACV. when the engine is fully cold it will be at its highest idle position and if it shoots above 2000 rpms for a second right after cranking it will rattle until the revs come back down. its just a matter of too much revs too soon on a cold motor. ideally on cold start it should shoot up but peak below 2k as to give the oil a chance to catch up. thats why the noise seems to always go away after a few seconds (at least for me it did).
never happened to me with the stock ecu, but I can tell you it used to drive me crazy with my ems in winter, till I figured out it and adjusted my idle % much lower above 1500rpm and lowered my starting rpm offset. you cant do that, but if thats whats happening it could be a sign that your idle is a bit off for some reason.

 

it just happens when i crank it on cold start its for a bit and my idle rpms stay past 1 and than drops to 800 but now something else came up now at idle it pulsates like misfiring but i hear it from my exhaust area in the mid and my engine moves a lot at idle so i dont know if its happen to anyone before. It usually at idle at a stop but when I accelerate gets deeper and goes away so please if anyone can help I would really appreciate it.