I also had this problem with my 2003 Nissan Altima. The car felt like a paint shaker, The engine was vibrating something fierce when it was idling. My Dad, thought it was the fuel pump and or injectors. He bought some cleaner to put in my gas tank but I browsed the internet and found this site. As others have suggested, It was unquestionably the intake manifold gasket that was defective. I spent $16.00 at O'Reily Auto Parts and 6 hours of my time to change it. Some of the bolts were in tight places and my arms are a little raw from the experience but it wasn't that difficult to fix. If you are experiencing this phenomenon, don't waste your money searching for other fixes.... It is the intake manifold gasket that needs to be replaced. I would like to say to the other posters, Thank You!, You saved me time and money on this fix. It is most appreciated.