99 OHV Plenum sensor

My 99 Sport 6 cly OHV has a high pitch wine coming from the plenum. When I tap on what appears to be a sensor about half way back on the driver's side of the plenum it disappears. What is this sensor? It has a black round cap about a half inch in diameter on the side of it that can be turned easily.

Thanks

Another case where a picture is worth a thousand words....

I'm guessing you're tapping on the Idle Air Control (IAC) solenoid.

-Rod