Things Mai knows about the Buzz (she may have told some of these things to others):
Lots of random folks on the street buzz, perhaps more than half. This causes there to be an annoying empathic interference pretty much anywhere Mai has gone in Chicago. Clearly these folks do not have any real empathy, but are still somehow broadcasting this sound, which feels like something is interfering with their thought processes.
All empaths encountered so far seem to hear the buzz too. They don't hear it as loudly as Mai, since they aren't as naturally empathic as she is (she has a higher EMP score). Like Mai, they've heard it all over Chicago.
The second worst source of the buzz was Max's monster, the Tulpa. At one point it buzzed so hard that this was part of it's mental assault on Mai in the alley in back of Marcus' bar. The crazy sorcerer Eli who created the Tulpa buzzed a lot too, third most of anybody she's met.
None of the members of the underground buzz. Not even Spense… until recently.
Mural Man (AKA the Prophet) buzzes a lot, and his buzz is strange, not like anyone else's. His buzz matches the mixed up buzz found in the neighborhood around the bar, centering on the cemetery.
Elspeth works at the bar in that neighborhood. She buzzes, but not like the neighborhood… but like everybody else in Chicago other than the Prophet.
Spenser brought in a case of Coca-cola, and started buzzing after drinking it. It's definitely possible that it's the source of the buzz in the common population of Chicago. Very popular drink now that it once again contains traces of cocaine.
The thing in the maintenance pit beneath the floor of the main hangar at the Snaky Airfield is now the single loudest buzzing thing encountered. Buzzed so hard it took down the whole team for a moment.
The box of stuff that they got routed from the Snaky Airfield had contaminated Blue Lotus in it. Taking this makes one buzz a ton, and it's clearly what they're adding to Coca-cola that's making lots of people buzz.
The Ra-stone at the Citadel of the Order of the White Circle buzzes.