Audible noise in DC-DC buck converter
The first question is, what is switching 5A or 7A at 100Hz doing to your +24V supply? It will be adding some ripple to it, can you measure how much on a scope? I'm guessing the ripple looks a bit like a square wave.

The second is, how much current do you need from that 3.3V supply? 0.1A, 1A, 10A?

Assuming less than 1A or less than 3W total, so the current drawn from the 24V supply is 0.1 or 0.2A...

Then you can feed the 3.3V supply from 24V via a low value resistor : 1 to 5 ohms, dropping less than an volt across the resistor - max 0.1 or 0.2W wasted power.

And replace C11 with a much larger capacitor - 100 to 1000 uf. Then R-C provide a low pass filter to reduce the PWM ripple, or at least slow its edges to a ramp, so that the regulator can respond fast enough to maintain control of the 3.3V output without "singing".