The problem: A loud engine knock, on the passenger side, around the valve cover, seemingly between cylinder 2 and 3. It does NOT do it at idle, and tends to start around 1900 rpm, and persists until around 3200 rpm. Cruising down the interstate at 70 mph, I can hear it. It primarily does it when the motor is under little to no load, and is noticeably worse when I let my foot off the pedal and coast. Weather and temperature do not seem to have any effect.

The video: Sorry for the fairly poor sound quality, it's difficult to get good audio on a phone with the fan blowing. Don't pay much attention to the video unless you see something glaring as I was mainly trying to focus on the audio rather than where the camera was pointed.
1999 Ford Lightning engine knock - YouTube

All spark plugs have been removed, inspected (well, to the best of my abilities) and replaced. They all have a orangish/brown hue to them. The pictures below show cylinder 1, 2 and 3...


(ignore the ghost)


The previous owner (my brother) said it has been doing it for a while, probably 5-6k miles if I were to guess, so I went ahead and made the return trip home from Maryland to Oklahoma in it, which amounted to around 1,300 miles. Thinking it was a lifter and not knowing when the last oil change was, I thought I would give a motor flush a shot to see if I could knock any sludge loose that might be blocking any passageways. Replaced the oil with Motorcraft synthetic blend, and Motorcraft oil filter, but alas, no luck... the knock persists.

The knock is definitely coming from the passenger side, and seems to come from around the number 2 or 3 cylinder. In an attempt to trouble shoot it, I disconnected the coil to number 3 as the motor was running, and the sound was noticeably quieter.

Again, all plugs have been pulled, inspected and re-installed. All plugs are snug (which seems to be the most common fix).

One final note, the motor has spit a plug before, about 30k miles ago. It spit number 2 and was supposedly fixed by Ford. I notice that it is the only one with a different coil too, which would lead me to think they only worked on the one cylinder.

Thanks in advanced. I'll continue combing the search to see if I can run across something else before I pull the valve cover. I also thought about checking the flex plate.

 

Well, first thing I would say is piston or bearing problem. Run a compression test first, see if you have good compression on all 4 and if it is even across the 4. If it is, you can rule out head/valve problems for the most part. Then it will be lower end time...