Turns out it was the heatshield around the exhaust. I got the car up on some blocks today to change the oil, filters, fluids, etc and also to take a look at what has been rattling under there all this time. The heatshield had rotted itsself loose from the 2 bolts all the way in the back. I was able to drill out the rusted bolts and put in some new ones with really big washers and now its sounds great. It was soooo embarrasing to be idling and hearing this terrible sound under my car.