Apple watch 9 41mm - Haptic tick every second while charging

Every time my wife charges her new Apple Watch Series 9, 41mm model, using the original base and 20W charger, the watch emits a series of haptic ticks every second, whether the screen is on or off. This sound is similar to a mechanical wristwatch ticking. It's bothersome, and I can hear it even when the screen is off. The noise ceases when the watch is removed from the charger. After experiencing this issue, we replaced the watch at the Apple store, but the problem reappeared two weeks later... I also have a new Apple Watch Series 9, 45mm model that charges silently without any odd noises. Me and my wife frequently swap chargers so it is not a matter of chargers, etc. Could this be a specific issue with the 41mm models?