Very Strange Noise.. HELP!

[HummerMann]

It sounds to me like it could be the cat O2 sensor giving you the check engine light. Unless someone ran leaded fuel in your engine to cook your cat, I'd be very leery of replacing the cat. The fuel line cleaning makes no sense at all to me - that's why I hate stealerships. They told me on a BMW I had 15 years ago the clicking noise at idle only was my injectors and they wanted $1500 to clean them. It turns out the Peake codes were for the VANOS circuit board and I fixed it myself for 130. It was the last dealing I ever had with that dealeship and BMWs, and I REALLY miss my old Hummer dealership here which had the best mechanics I ever dealt with.I would definitely get a second opinion.