Spark Knock noise?
I noticed that same issue when I was working on another problem, and it ended up being some funk stuck to the fuel filter in the fuel pump assembly. If you scroll down about half way on that first page, you'll see the pics of what it was, and another link to how to remove the fuel pump assembly.
Taking out the fuel pump is fairly easy, just be sure to take out the fuel pump fuse in the fusebox hidden under the plastic cover next to the battery under the hood to de-pressure the fuel system....