I have been noticing an intermittent noise when I start the car after it has sat for more than a few hours. It is definitely coming from the engine compartment and sounds like when I was a kid and would clothespin a baseball card to the spokes of my bike. It speeds up when you step on the gas so it is linked to the RPM'* of the motor somehow and goes away once the car warms up. I have had the hood up and I don't see anything out of place. Any ideas? If you need a better description I can try and do better but that'* pretty much what it sounds like and it goes away after about 5 mins of driving and doesn't do it everytime I start it.
exhaust was my first thought. There is a local radio show here once a week where people can call in and get advice on problems and a guy had a similar issue. They said that what happens is that there can be a small leak in the exhaust (in this case I would assume the manifold since the noise is DEFINITELY coming from under the hood) but once the motor heats up and the air gets hot the metal expands and seals the leak. I just can't figure out why it doesn't do it EVERY time. I'm hoping thats all it is because I don't think that is going to cause too much damage and I can save some $$$ before taking it in to be fixed.
From what I can tell, it is coming from the passenger side of the engine compartment. It sounds almost like it is coming from the pulley system but I didn't find any obstructions and that wouldn't explain why it keeps coming back and going away.