OK, I'm at wits end trying to track down the random noises that this car has been making over the last couple of months, but the last two days have taken the cake:


Yesterday morning
It's very cold outside, around 15-20 degrees. I start the car, expecting to hear the usual elevated tick from the engine, as it does on cold mornings. However, I hear what sounds initially like a horrible rod knock.

This knock settles down to more of a slap within 20 seconds, but there is a constant moderately loud slap sound going on. In addition, the exhaust smells like crap. I towed the car to the dealership. they reproduced the issue, but once the car warmed up, it went away totally. No codes thrown.
Last night
Go to start the car (again cold start), and I get two new noises:

1. Low volume slap while engine is idling, smooths out some when warm, but still there
2. Before I started the car, I turned the key to ACC On. Instead of the usual fuel pump priming sound, I heard three fast squeak sounds, pretty high pitched. Almost sounded like a high pitched "wrong answer" buzzer
3. When idling, if I goose the gas, then let off of it, I hear the same three squeaks as the engine revs drop.

Dropped the car off at dealership, and the service techs heard the noise. they started it this morning, and they now say the noise is gone, and they see no issues. I'm desperate for some help. any ideas?
NOTE:
All fluids are fresh, air filter is clean
No service lights on at all

 

Sounds like VTC timing chain noise that all 3.5 engines get eventually. Search around for more information. And look at your oil level.

 

You probably need a new timing chain tensioner and a new fuel pump. Foul smelling exhaust is indicative of a bad catalytic converter, but I would only worry about it if you fail emissions.

I would only worry about this stuff if the problem persists or gets worse.