What is the hole under the black knob on the carbureator for?

[CustomGT]

On my 64 Fairlane with 289, on the passenger side under the black lean knob, is a hole the size of your finger. It points strait down.

If i put my finger up it, the throttle plate closes, so I assume there is a spring or actuator in it.

The problem is, is that it whistles really loud, it sounds just like a belt squealing. It did not do this prior to changing the water pump, so Im not sure what happened. If I put my finger over the hole, the sound quits.

Should there be a plug or a hose on this hole?

[CustomGT]

i guess it is for working on that spring or actuator.

i found a rubber piece in my garage and just stuck it up in it. the whistle is gone now...i guess i fixed it, seems to run a little better