I'm curious why its only the left head. Are those heads asymmetrical? Most aren't, so both heads should make the noise.

There were some Duratech 2.3 heads that left the assembly plant without a machining step that drills an oil passage for the valvetrain. Oopsies, no oil makes for more than just "the tick"

I believe Ford caught them before any made it into vehicles, but some guys are buying them through the Ford "scrappers" and have to watch out for a certain date code.