Well I've looked/listen to numerous videos and I can't find anything that replicates the noise that my 1999 XK8 is producing.

Video Link

The knock has a consistent wave pattern (comes-and-goes, versus knocking every rotation). Idk if it's correlated, but this knock (and rough idle) came after replacing the MAF.

My troubleshooting process:
Lean codes (P0171 & P0174), but smooth idle and no knock --> Seal all vacuum & exhaust leaks, new MAF, reset ECU --> Drive Cycle --> Rich codes (P0172 & P0175) --> MAF error (P0101), Rich codes (P0172 & P0175), rough idle, engine pattern knock --> reinstall old MAF, reset ECU --> rough idle and knock are still present  --> clean and test MAF plug contact points (all good) --> test idle without the MAF plugged in --> same rough idle and knock

The car knows the MAF is being plugged in because it does temporarily correct the idle when I plug it in as the engine is running, but it quickly reverts back to the rough idle and knock shortly after.

Any other ideas?

Why did you replace the MAF in the first place?

It sounds as if unintentional mechanical contact is happening and that is causing the knock sensor to !Removed! the ignition timing to protect the engine but causing a rough idle.

Have you checked the Lambda (pre-cat) outputs? If they are old and/or failing, there's a good chance they are giving a weak/lean mixture signal to the ECU which has been known on some cars to give a "red herring" fault by fooling the ECU into thinking the MAF isn't responding. Also have you checked fuel pressure? If the fuel pressure is low, that could cause a rough idle and potentially throw a MAF fault.

I suspect the fault lies somewhere other than the MAF, if the Lambda sensors are 10+ years old or 100k+ miles there is a good chance they're past their best. They may not show as faulty until they completely fail, meanwhile your fuel economy and genral running will suffer so worth checking their outputs.