The W22 (At least mine) also has an electric booster pump on the brake system. I do not hear it when the engine is running, only when the engine is off, this does not mean it's not running when the engine is running. It simply means I can not hear it.
It's not a high pitch though, very clearly what it is, Same sound my Lumina makes when I pull the breakway plug out of it
(Same kind of pump)
Vaccume leaks are hard to spot, But they fall into two areas
1: Does not matter if you put foot on brake. (Check all hoses and connectoins)
2: Only happens when you put foot on brakes (Check vaccume booster module, nowhere else)
My lumina did the latter... OUCH