The problem is that my 1200 watt conventional heating element is creating a mechanical buzzing noise. Shown below is a schematic of my setup:
PD4 is the output from a 5v microcontroller and is triggered high based on an interrupt from a zero-cross over circuit.
AC hot out and AC neutral are connected to the conventional heating element. The triac is ultimately used to control the duty cycle of the heating element.
From what I've read, the mechanical buzzing is due to the high change in current vs change in time (di/dt) caused by varying the phase of the triac. The step in current can be as high as 15 amps.
Based on some research this problem sometimes surfaces as "light bulb ringing" in light dimming applications. Some solutions people have proposed is to use a choke inductor.
I setup a 39uH inductor in series with the hot load line thinking it would reduce the di/dt of the circuit however it made no change to the buzzing.