High pitch noise when resistive heating elements turn on
It's possible the noise originates in the heating elements. Triacs, depending on the way they are driven, can turn on sooner or later in the cycle, if they turn on later (relative to the zero-crossing) there is a sharp increase in current at double mains frequency, and an attendant force impulse from the magnetic fields that can stimulate mechanical resonances.
If your heater construction is subject to that kind of resonance, then you could have audible noise. Incandescent bulbs controlled by triac dimmers can emit noise for this reason. There is a fundamental frequency at 120Hz or 100Hz, but also a relatively high frequency pinging that results from the mechanical resonance.