Octane Knock

[Nicholh]

So I have a 99 town car, 4.6Liter with 201,000 miles. No smoke, check engine light or anything. At highway speeds, the engine makes an audible knock, like a low octane issue. In the past I have used 87 octane and it made this sound. I have always used 93 octane ever since, and it has not made that sound...until now.
I unplugged the MAF and the sound is gone, gone GONE. The check engine light comes on and the mpg has dropped. So I replaced the MAF with a new Motocraft one, but it still has the knock, but only at highway load and only with the MAF plugged in.
Any advice here would be terrific.