I have a sound that seems to be coming from behind the dash on the driver's side. It occurs at WOT only and it occurs in 2H only (not in 4A at all). It sounds like a bunch of keys dangling from a key ring and being jingled a little and the sound lasts for about a full second. At first I thought it sounded a little like the buzz from an electrical issue but it sounds more mechanical. As I kept thinking about it, I thought maybe detonation but it's too soft and doesn't seem to be coming from the engine bay. I don't have coins lying around in the truck to rattle and it only happens at around 5K RPM give or take 500. I took it to my Ford dealer and the shop foreman rode with me an heard it himself. He thought it was a check valve but didn't seem too confident. He had the tech replace the check valve. Didn't do a thing. Haven't taken it back to him yet. I've read the vacuum check valve and "grinding problem" posts but those don't seem to be the issue. If you have any ideas about what it could be, let me know.