If the buzzing is coming from the first stage, I had the similar problem once when I got my old blizzard back from servicing. In the water, look for a greater amount (and perhaps larger bubbles) coming from the dry-bleed port.
The problem (in my reg) was that the tech that assembled and adjusted the reg damaged on of the internal parts. I think the part was the piston seat.
Also, from the Sherwood service manual:
D. Humming or Buzzing During Inhalation:
Cause: Harmonic resonance between the springs and the mass of the piston.
Solution: Disassemble the first stage and flip the main spring over. Change the position of the piston in its bore. If resonance is still present, install a new piston and spring.
Either way, take it back.