1998 F150 4x4 5.4L.....pursistant knocking underneath the engine originating from the oil pan/transmission area. it dies off a little at higher speeds >55mph. All other operations: idle, temp, pressures, acceleration, etc are fine. No check engine lights on. Oil pump on the way out, maybe??

 

My wild guess would be that you need a new engine. You might want to check the flexplate to make sure it's secure. If you could post a video of the engine knocking we might wild guess a little better.

 

How many miles on it? What type of oil and filter are you using?