I am getting a faint rattling noise lately under light throttle, hot or cold.
Engine runs fine without this rattling when parked. It does not happen all the time, but does happen frequently. Car is a 07 i-touring, automatic. It sounds as if you put in 4th gear in a manual car when you are at 20 MPH. Does anyone of you have this problem? Just wanted to have your thoughts before I have to bring to a dealer. Car is 20000 miles, no hard driving, commute use.
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cant really tell by hearing it myself but if i had to take a wild guess it could be your heat shield for your exhaust manifold

 

The area around the bolts on my exhaust manifold heat shield, then around the exhaust pipe heat shield, and then around the welds on my cat converter heat shield rusted away and so the shields were rattling.

20k miles is a bit early for that IMO, but it shouldn't be too hard to check those areas. And your symptoms (quiet rattle at low speeds) are real similar to mine. If you have one of those problems, I took care of mine as follows:

Exhaust manifold: put a ceiling fan hanger bracket between the bolt & heat shield.
Exhaust pipe: put big washers in, then eventually removed the heat shield (and didn't park on grass)
Cat: replaced the cat (due to CEL indication, not just b/c the heat shield)

 

You might also have the bad, dreaded RS engine mount failure, though I thought they were rare in the 2.0L's... common in the 2.3's.

 

I had that problem once and it was the alternator. Sometimes when the belt is lose or the alternator is going bad it starts to make a rattling noise (sounds like peddles in a can) when i accelerated.