Hey I got a F150 5.4L And every so often I get a rattling noise (kind of like a rattlesnake rattle) coming from the DS firewall in my truck when I start to go up a hill. The noise is quite evendant until the truck shifts out of OD then it fades, then goes away completely once I've climbed the hill and have resume coasting down the highway in OD. I don't have any kind of fault lights on what so ever. Anyone got a idea of what this might be? truck doesn't lack power or hesitate at all!!

 

Probably spark knock. Use a higher octane fuel and see if it goes away.