Back-story:
A couple days before I got my exhaust manifold replaced, the truck sounded like the leak was getting worse and worse. When I got it back, there was this "tick" which sounded like the exhaust leak, but we (myself and the mechanic from Ford) could not find the source of the leak.
He thought that perhaps the truck had "developed" a tick. I wasn't in agreement, since the engine has always been smooth and quiet. I thought perhaps my recent switch to M1 0w30 loosened something that got caught in and was causing an HLA to stick. But it REALLY sounded like an exhaust leak......
Well, lesson learned for both myself and my Ford mechanic friend:
When a Modular has a spark plug that has loosened off, it makes a noise that sounds like an exhaust leak with no discernible leak point. It sounds like it is coming from the manifold area; there is no real evidence of noise from above. But you cannot pin-point the location of the noise.
So, driving to work today.............. POP!!!!!! pfwah pfwah pfwah pfwah "oh no" I say to myself, as I figured that is what it was.
Pulled into Mack's Milk parking lot, popped the hood. Couldn't see anything immediately amiss.
Called my wife to bring me my tool set.
When she arrived, I hopped up onto the rad support and started digging around. I found the top of the coil for #4 cylinder (very back cylinder on right-hand side, the one with the coolant tubes running over it). Then I found the plug, it was sitting on top of the intake manifold right at the very back. There was part of the coil still bolted to the intake manifold. The boot for the coil was also off, stuck down by the runner.
Took the Focus up to Ford, and talked to my buddy Mike at the parts counter. I told him I was hoping it didn't take the threads with it (there was no evidence of thread material on the plug) and he laughed and wished me luck. And I picked up a new plug and coil.
Went back to the truck, stuck the plug (DRY this time, no anti-seize this time around!) in the plug socket, and down the hole. Threaded right back into the hole and snugged up firmly without issue. Installed the new coil and I was back in business.
This was the hardest plug to do when I did them. And the last one I did. I don't know if this contributed to why it popped out, if it was the anti-seize or maybe a combination of the above. But she's back in now and my "tick" is now of course completely gone
Lesson learned!