I have the same issue on my '02 XLS 4x4.
I took it into the dealership and asked about it. They thought the timing chain was going to have to be replaced and wanted my approval to tear it down basically. It was under warranty (extended) so I said go right ahead.
But, my extended warranty people gave them some grief so they never did it. So, they found another Explorer... same year and model... and test drove it and it produced the same noise. So, he comes back to me and says they are going to classify this "noise" as a trait of this engine.
"Whatever" I'm thinking... because I notice the noise lowers after, yes, an oil change. What's even more interesting is that when I had my transmission flushed, that noise completely disappeared for about a week, then came back. About a month ago, I had my fuel filter changed. That also took care of the noise.
After the fuel filter change, I still hear it but it is far less noticeable.
I stll don't know what to do about it and hope it doesn't cause me any problems in the future. My warranty expires July 31st....
Oh, and rolled over to 66,000 miles as of yesterday...