Annoying Noise?

so yea whenever my truck is idling it sounds like there is a grinding/gurgling noise come from the exhaust, or at least around the middle of the truck underneath. I was thinkin it may be a problem with the cat or muffler, both which are still stock, but im not sure. Almost a year ago I accidentily filled up like half my tank with diesel fuel...stupid as hell, long story, anyways i was wondering if anyone could tell me what the problem is? Someone suggested it could be the pinion angle but it only makes the noise while idling. Thanks in advance for any help.