I have an engine that starts knocking after driving for ten minutes at highway speed. I drove 30 miles last weekend, the first 20 was just around town and the last 10 was down the highway. The engine started knocking after about 8 miles on the highway. The sound got faster when I pressed the gas. I slowed down and returned home (2 miles) and before I got home the knocking sound was gone, it faded away. I replicated this exact same scenario twice now.   
It’s an 87 ford bronco 2 with a 2.9 that I am trying to save from the scrapper. I don’t want to throw anymore money at it until I know if the engine is toast.   
Another piece of information that I am not sure is related is that a few months ago (before the car was road worthy) I removed the spark plugs to check the engine compression. After I put everything back together and started the car I heard the same or a similar noise from the engine, idling in the driveway. I removed all the plugs and reinstalled, then started the engine and heard the same noise but then it just faded away after about a minute. The video is from when I checked the compression, right before the sound faded away, it was louder. I have not captured the sound while driving yet. Any suggestions on what I should check first? 

Engine knock at high speed