I have a sound 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. 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 between 5-6K RPM and only at WOT . 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 about his diagnosis. He had one of the techs to replace a 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 that doesn't seem to be my issue. I couldn't get the video file size small enough to upload, unfortunately. If anyone is willing to PM me your email address, I'll send the video that way. If you get the video, you'll need to turn your volume up pretty high to hear what I can only describe as keys being lightly jingled at around 5500 RPM. The jingling sounds louder in real life than in the video. The sound occurs consistently. Again, it sounds like it's coming from under the dash and I feel NO VIBRATION from the drivetrain when it occurs. Doesn't matter if the engine is cold, warm or hot. I'd like to fix it before something serious goes wrong. Let me know what you think it is.