2003 pontiac grand am gt engine noise?

I bought the car new in 2003 and have taken great care of it. I have had all scheduled maintenance done and replaced those things that have broken down over time due to "wear and tear." I now have 165,000 miles on it…

Last week, I was idling in traffic behind a Honda and heard a loud, arrhythmic banging/gurgling sound. I thought, "Man that Honda's in terrible shape;" it turned left and I realized the sound was my car. The noise has not stopped. I can hear the noise while sitting still but not while driving/accelerating. I would say it started that day but I didn't hit anything or hear anything break…

I checked my coolant level and added a little bit just to be safe. I replaced the radiator cap since I read that the gurgling noise might be air getting into the coolant system, and because the noise seems to be coming from that general area of the engine. I had the oil changed on schedule about a month ago and the level is currently where it should be. The air filter's a little dirty but there's nothing loose in there. None of the hoses or belts appear to be worn. Since I've had the car for 11 years, the catalytic converter, fuel pump, and intake manifold gasket have all been replaced.

What is making this noise, and how much am I looking at?

In agreement with Bo, sort of hard for us to hear this noise. I suggest either a repair shop or any GM Dealership. Can be anything

Sorry to say you'll have to take it a garage to let them hear the noise. You'll get several different answers but you'll get the real answer by someone actually listening to it