No, the knock sensor hasn't been checked yet. The service tech said it would *probably* throw a CEL if it malfunctioned, but I suspect you might be on the right track in your diagnosis. I may have to break down and buy a scanner. As technical as cars are today, it'd probably behoove me anyway for future repairs. Besides, I could keep checking for that P0402 on the Miata.
As for the head gasket, I have some doubts as the car runs like a champ. Wouldn't performance be degraded if the head gasket was leaking? I'm asking, not refuting.
Thanks for the responses, guys.