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If its actually the engine and not some relay clicking.. Probably a valve lifter.If its actually the engine and not some relay clicking.. Probably a valve lifter.
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The 3.4L engine has special pistons that are designed to expand as the engine heats up. When the engine is cold, the pistons make a knocking sound that resembles lifter noise or piston slap. As long as the noise goes away after the engine warms up, don't sweat it; this is normal.
If it goes away after the engine warms up it is likely loose pistons or worn piston skits. As long as it goes away after the engine is warm it won't hurt anything. It is common on engines with some age on them. When the engine get hot the pistons expand and the noise usually goes away.
4.0l's are notorious for valve rattle on cold start-up. There is really nothing you can do to make it go away short of completely replacing the valve train in the top end, and that isn't a permanent fix. It's is an top end oiling issue from the factory. Just don't race your engine on cold start-up until the rattle stops.
The rattle in the transmission sounds to me to be more of a loose dust shield/heat shield around the transmission or exhaust than a mechanical issue.
It sounds like you may have some sludge in your oil. When the engine is cold the oil is thicker and takes a while for the oil pump to pump it through the engine. The next time you change the oil. Do an engine oil flush first to clean the inside of the engine and ports. If this does not work you might have to have the oil pump checked. There is no need to worry as long as you have oil pressure and the knocking stops. I hope this helps. Don't forget to rate this answer.
Sounds like faulty engine temp sensor. When it drops to cold, the engine control unit picks this up and alters the fuel mixture accordingly. The ECU will be making it a very rich fuel mixture for cold starting, hence very poor running when engine is hot.
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