I drive a 2006 Chevy HHR LT 2.2L vehicle.
My question is why or what is causing a loud rattling sound while idling?

 

If you have a lot of miles on your car it a strong possibility the timing chain is to the point of replacing.

 

Too vague of a description to guess.
Where is the noise ? In the engine area ? Rear of the car ?
Have you even looked and listened under the hood ? Come back with a better description and we'll try and help.

 

Many rattling points to investigate, like.....
the above mentioned rattling
the air filter box bracket
the flex pipe starting to break
the sway bar end links
the brake pads
the plastic hub caps
the bumper covers
the plastic inner fender liners
the air filter box grommet missing

just to start your investigation

 

Also check the Heat Shield if it is a tinny sound . But a better description will help. The gear heads here can better guide you

 

It's sounds like its coming from either the muffler or the catalytic converter. Could be the flex pipe as mentioned. It sounds like an exhaust issue. I took the car to autozone to get it checked and it gave a code for a bad fuel tank pressure sensor. However, I went ahead and replaced the fuel cap and the canister purge valve solenoid hoping maybe it would shut off the check engine light but to no avail, its still on. Im thinking maybe it has to do with the exhaust.

 

The car has about 160,000 miles on it. I bought it towards the end of 2015. About in October. It had about 96,000 miles on it then.

 

Again, please post that code that AZ pulled, they usually interpret them much differently then we do, why? You ask? Because they are selling parts, we are offering experience and expertise to resolve the problem as inexpensive as possible.
I chased a P0420 code for some time, turned out, it was a wiring harness issue, not new O2 sensors or a new catalytic converter as the parts store guy told me!

 

airbox ?

 

3 codes. 1.P0455 2. P0326 3.P0442