I recently fixed a vacuum leak in my w202 that made an annoying high pitched squeal under the dash. But I don’t think that is what is going on here.

So your part is some kind of electrical switchover valve. It takes electrical signals from a controller (hvac) and connects or interrupts vacuum flow likely to flap actuators.

I would start by disconnecting the wires (one at a time if possible) and seeing if it stops. Also disconnect vacuum hoses one at a time and seeing if it stops. This can tell you if it is on the vacuum or electrical side.

My gut feel is it’s on the electrical side. Sometimes a bad driver circuit can make a solenoid buzz like on an old style doorbell buzzer. So your problem may lie in what is driving the switchover valve.

A good way to verify this is when it buzzes probe the wires (with a pin poking through the insulation) and check their voltage. Either with a meter or scope. You might see a jumpy signal. You should get 12v constant voltage or 0. That’s the way my w126 works. When these switchovers fail usually the coil opens. That’s why I think your issue is actually the electrical drive signal.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles