It's the regulator. I know you've replaced the regulator. The new one is squealing just like the old one.
A gas regulator contains a diaphragm, a spring and a needle valve. When the outlet pressure against the diaphragm falls below the design pressure, which is set by the calibrated spring, the diaphragm allows the needle to withdraw slightly, admitting more gas and restoring the design pressure.
When pressure and flow are constant, the diaphragm and needle reach an equilibrium position that lets just enough gas through to maintain that pressure. But at one particular supply pressure and gas flow rate, any random vibration in the environment will cause the diaphragm to resonate at its natural frequency. While resonating, the needle is slightly opening and closing the valve, and the varying flow of gas reinforces the resonant vibration, prolonging it indefinitely.
Skeptical? If you can do so safely, light the oven, wait till the squeal starts, and then whack the regulator with a wooden spoon or some other object that will cause no damage. I think you'll hear the squeal pause for a moment, then resume.
The squeal is not caused by turbulence, a restriction or a bad jet. It's caused by having the bad luck to have a gas main pressure and oven jet flow rate that's exactly right to reinforce the resonant frequency of your regulator's diaphragm.
If you had an oven jet one size bigger or smaller, the regulator would not squeal, because the flow rate would be different. But changing jets is not a safe solution.
If you can find a different model of gas regulator with the same design pressure that fits in the available space -- presuming that it has a different size diaphragm -- it won't squeal.
