more on (moron) fuel pump whine

[brumleveb]

I'm wondering if the fuel pump noise (whining) I'm hearing is some sort of backpressure/venting issue. I noticed that the problem stopped a few weeks ago when I only filled to 3/4 tank. Last week, I filled completely and the whine returned. Last night, while the truck was in the driveway, I took the gas cap off and put it back on. I don't remember hearing the whine this morning on the way to work.
Can someone else try this and see if they get the same result? My truck did only have a 1/2 tank last night. It is an '04 2500HD with the 6.0L engine.