Hey PT, I have two WF-138 chargers and just tested them both for you. I mostly use them at the 3V setting for my AW LiFePO4s and never hear a buzzing sound but decided to set them up at 3.7V and attempted to charge one partially drained AW 16340 IMR battery. I got the buzz sound from both chargers. It wasn't really bad or really loud but definitely noticeable, especially when my ear is about a foot away from the charger. If you've heard others say that it's normal to hear the buzzing sound at the 3.7V setting I wouldn't worry too much. The odds of all three chargers between yours and mine having a defect would be pretty slim... If someone out there has some real data conflicting with these findings, your advice would be greatly appreciated.