When i start up my 2007 4.0 Ranger when the engine is cold. i get a few very random "knock" sounds from the engine. First one will be after its been running for 15-20 seconds, then again after about 1 minute, and maybe another one 30 seconds later. strange to be honest. im wondering if its possibly something in the AC compressor/clutch?

 

Select OFF on the climate control before starting, no knock then it was AC compressor

Try priming the oil system before allowing the engine to start
There is a thing called "Clear Flooded Engine mode" on all fuel injection computers, Fords as well
Turn key on
Press gas pedal down to the floor and HOLD IT DOWN ALL THE WAY
Turn key to START
Engine should crank but NOT start, the fuel injectors are turned OFF
Crank it for a few second, count to 5 maybe, then release gas pedal and injectors will start, and so will the engine
See if knock is gone

I use this every morning on my high mile 4.0l because of the high miles the oil drains down after 6 hours or so
So I prime oil system before letting it start

If your engine starts with gas pedal to the floor then your throttle cable has stretched so you no longer have FULL throttle, google: ranger throttle cable mod, its an easy fix

 

I get kind of a semi loud clicking sound from my engine during a cold start, but I would not call it a knock. I has done this for a long time, maybe since new. I have no idea of what causes it but it doesn't seem to be a problem. It goes away eventually and does not seem loud enough to be of concern.