I have a 2011 f150 5.0 1st gen with 116k miles. Dealer maintained has developed a sound that I cannot find . Not ready to take it to the dealer yet . Only does this when it’s hot , in drive and foot on the brake. Goes away with RPM increase . Distinct slapping sound ! Not the flex plate , already pulled it , it was easier than getting into the motor .Let me know your thoughts !!