Just bought a Nissin Di622 flash for Nikon.
Didn't notice the humming sound at the shop as I guess it was too noisy there.
After powering it up, it has this high pitch humming sound which happens every 3 seconds continually. I tried changing batteries with fresh ones but its still the same. The manual never mentioned a thing about the sound.
Its a bit like the flash charging sound after a full blast. However, its continuous happening every 3 seconds or so.
Anyone else know this phenomenon?