Rattle noise from engine at start up for a few sec.and goes away. Took the car to dealer, and was told its normal.

start the engine up and keep an eye on the power steering pump. if you see the pulley wobbling, shut the engine off and replace the pump. I dont recommend reusing the same pump because if it is wobbling, then that wobble will damage the shaft seal. another highly cause of that rattle might be just an exhaust shield, as you drive the exhaust expands with the heat and the noise goes away.

hydraulic tappets ?
if the noise goes away.... thats normal

if the noise didnt go away - then that would need looking into

get it into a garage before something really goes bad then you'll be looking for a new beamer

READ THIS (happy ending so far...):
audi a4 2L turbo quattro, manual transmission with 98,000 KM. Perfect conditions, no oil leaks whatsoever.

Left the car off for two weeks in very cold weather (-10C down to -25C). When I restarted, it started immediately but with heavy rattling noise due to camshaft adjuster not getting enough oil pressure. After 3 minutes of warming up (outside temp.: -12C) at very low rpm (below 2,000rpm), I start moving the car. Still hearing heavy rattle noise. As soon as I pass 2,000 rpm, I get a scary red warning, low oil pressure. Turn off the engine immediately. In 1 minute, I re-start the car and the engine lights check ok, so I drive in first gear super slow (1,200rpm) hearing the same rattling noise. After another minute, finally the noise all the sudden disappears and the car starts running just fine.

Called Audi dealer and they told me that in extreme weather conditions, I should warm the engine for 10-15 minutes if necessary before start moving.
Take away: luckily, I didn't screw up the engine since I was using it at extremely low rpm (easy to do with clutch and manual transmission) until the oil reached full pressure. I am pretty sure now that the problem was leaving the car off for such a long time in freezing weather. All the oil flows back to the bottom, and when I restarted it, the cold oil with high viscosity took a long time to reach all the parts of the engine, specially the camshaft adjusters that are actuated by using oil pressure. Next day, the car started at -9C perfectly without making any noise (as usual). Basically, the longer you leave your audi in cold weather, the longer you might hear the rattling noise until the oil reach full pressure inside the engine. My car took 6 minutes to fix it after couple of weeks in cold weather. This makes sense to me; in fact, in the past I only heard the rattling noise after leaving the car off for a couple of days in the winter.
So far, this has had an happy ending....

Your cataylic convertor is going south thats the rattleing noise eventualy it will clog you may get a couple of years at best but you need to start thinking about replaceing it

explain rattling noise. you can take off the fan belts. start engine briefly to see if engine stil makes noise. that will tell you if its engine or the acc.

this is nothing, do a searh on chrysler rack and pinion problems and you'd be blown away at what you find. Also chrysler engine sludge issues. This is why we still stick with a BMW. A a mechanic we have seen several customers get rid of the cobalt ASAP. Just like you they felt like they bought a lemon. I'd hate to think that both dealers didnt catch something like maybe loose bolts or rattling panels or something.

Some noise from the AC pump is normal, but if the noise is changing or gets worse then the pump is probable going bad and will need serviced or replaced.

It's normal. When the car has sat, the oil drains down to the pan. On startup, you're starting an engine with no oil pressure (any car is like this), but once the pressure pumps up after a second or two, the noise goes away. Don't worry about this - it's normal.

I have the exact same car and it makes the exact same noise (though I've always called it "2 plastic cups banging against each other"). I was also told that it was the ABS/Traction control system and that it was a normal problem. I have had the car since it was new and it has always made this noise. There have been no apparent complications due to the noise or otherwise.