I have a 03 Ranger 3.0 truck has 118,000 miles was running fine no problems all the sudden it started to misfire on cylinder #5 so I did the obvious changed plugs and wires, this changed nothing I had to continue to drive the truck but then it started making a hollow knocking noise I crawled under my truck and the noise seemed to be coming from the exhaust pipe I listened at the oil pan “used a piece of wood” sounds like a sowing machine listened to the exhaust at cylinder #5 and it sounds like a hammer is hitting it from inside the engine. I pulled the plug and tested compression and it was 51 pounds. So yesterday I pulled the head on drivers side expecting to see a broken valve or valve spring but I did not see anything obvious. I am taking the head to a machine shop today. I had a mechanic tell me it’s the rings or a wrist pin I think this is a long shot I have been working on cars for a long time both a shade tree and in a shop and have never seen a wrist pin go bad, and that would not cause low compression or a misfire. I also think rings id a long shot because 50 pounds of compression seems high for broken rings to me. I do feel that the problem is in the head but the knocking noise threw me for a loop. I don’t feel this truck needs an engine any help would be appreciated... thanks

 

I can never remember the years that are effected, but you may be experiencing Ford's soft valve seat problem.
The seat gets hammered to the point where the lifter can't compensate for the lessened clearance and you get a valve leak.
If you remove the valve on #5, it will be evident on the valve seat and valve surface that it isn't closing properly.

EDIT:
The years that Ford had the valve seat problem was from 2004 to 2006, but if yours is a late 2003, it may be affected, best to look into it and take a close inspection of the number 5 valve seat.

 

A non-firing cylinder knock is troubling, connecting rod knocks louder when cylinder is firing, doesn't go away but much quieter when you stop spark to that cylinder

50psi compression, did you check another cylinder to confirm it was 150+ psi that day?
Just to make the 50psi reliable for that time of that day with that pressure gauge

The recessed exhaust valve issue with the 3.0l would never cause a noise, just a misfire from lower compression

I would check piston heights on that bank, and how much movement at TDC and BDC on all 3 pistons

And of course the cylinder walls

 

So dropped the head off at machine shop today and the noise and the low compression is becouse of recessed valve so took off the other head and machine shop should have them ready to go Friday head gasket kit and new head bolts will be here tomarrow

 

 

Odd why any machine shop would fix that, although it is certainly possible, labour to do all the seats would usually exceed the cost of re-manufactured heads.

 

Good news but what was making the "noise"?

Was #5 valve burnt?

 

The cost to completely rebuild both heads new everything $400 remained heads were around $174 for cheapest but shipping would have been the same and I trust our machine shop as I have seen his work. The noise was the valve banging around in the head

 

I got brand new heads complete $359-99 .. shipping $59-45.. tax $29-70.......... total $449-14 AdvCylinderHeads.com
I was told by the machine shop that re manufactured heads are junk they wont last long

 

I would for sure check your lifters in that cylinder, both the exhaust and the intake, might want to even replace them for good measure.
If the lifter is stuck or not working to it's full travel, that would also explain the noise _ although not necessarily a knocking noise, but a very loud tick