I have a 1998 Expedition with 5.4L & auto Trans. For several months now it has had an engine knock. The knocking noise occurs when I'm parked and I rev the engine and as it just starts to decelerate you hear it knock briefly, also when I'm driving as long as the engine is under load it is ok, but if I slightly let off a little throttle so the engine is under very little load or no load you can her it knock, almost like an old engine that wants to ping with bad gas (I'm dating myself with that comment), however this is definitely not a ping (detonation) this is a faint knocking sound. Once you give it a bit more throttle or start to pull up a slight grade or a hill and the engine becomes under a load the knocking will instantly stop. To me it sounds like a bad rod bearing but others have told me it could be something else. The vehicle has 123,000 miles on it and runs smooth with plenty of power. The previous owner did a fair job of keeping up with oil changes but I know they didn't change the oil religiously and they knew nothing about using the proper oil or filter.
What are your opinions as to where I should start the diagnosis?
The previous owner said 2 shops had just listened to it and told her she needed a new engine, but I'm a bit skeptical. What do you guys think?

 

When were the plugs changed? My 00 5.4 blew a plug at 90k and it made a light tick/knock similar to what you are describing. May be worth torquing them.