The freq valve is supposed to buzz. You can't tell just by listening that it is operating at the correct frequency and whether it is leaning or richening. To do that you must attach a dwell meter or a duty cycle meter and observe the dwell or duty cycle.

The mixture correction done by the controlling of that valve is throughout the rpm range. What does it do if you unplug the O2 sensor?

What does it do once warmed up if you grab the lead the O2 sensor plugs to and touch the battery + with other hand? If you touch ground?

These touch tests aren't a joke, the battery pos post test wil send higher (over.5V) voltage to Lambda ECU and the system should lean down and engg should run worse. Touching ground will reduce the V to ECU to below .5V and the system should richen up and it should run better and maybe even go too rich.