I've been wracking my brain trying to find out who exactly Detective Noel might be and I think I have a vague hand wavy idea.

I know people want it to be Parker, Kayne, or even the KIY but I think Arthur would have recognized Parker's voice even if John struggled to recognize him, Kayne was pointed out to have never known about the lighter, and the King seems like a long shot, but not altogether impossible. However!

Way back in season 1 we learned that the book that had once held John was actually addressed to Roland Cummings, a retired private investigator who owned 13 Mosby Avenue (Arthur and Parker's office) and as of now is MIA after the police found the body of his wife in the freezer and the neighbors suspected he fled to Canada (Part Two: The Missing Girl). He's also the father of Amanda Cummings, one of the missing girls who was later killed by the King (because of Arthur, whoops). Now, his office was leased out to Elijah Strong and his partner, Teddy Caine before being taken over by Arthur and Parker. Elijah had committed suicide in prison and we later find out that the Butcher was the one who actually killed him. As of right now we have no definitive answer as to what became of Teddy Caine.

Arthur's engraved lighter came from the desk of their office.

My reach is that Detective Noel is either Roland Cummings or (less so) Teddy Cain. I'm leaning more towards Cummings simply because he had seemed the most knowledgeable about The Horrors™️ and was in fact sort of the catalyst to the entire series being the original receiver for John's book. And it's also possible he and Arthur never really met. It would also make sense as to why he's working under a different name because he would still be under suspicion after what happened with his family near Arkham. Teddy Caine I'm less sure about because all we know is that he worked at the office for a time and had a run-in with the Butcher.

Detective Noel could also literally be none of these people or someone else entirely and I'm just thinking too hard. But ooh the possibilities! They make me [stick figure gore]