what do I do to a whistle that is too airy?
Let some of the air out?
Sorry, I couldn’t resist. 
Hi
Can you be a bit more specific 
Cheers
Depends on which whistle, and how it is constructed.
If it’s a Clarke Original,-squash the windway.
cover 20% of the blow slot with bees wax
It’s a PVC whistle, Guido-style.
You need to reduce the height of the windway. Guido has some hints about this on his web. Basically shim the top down is what he suggests. With a whistle that already exists, that’s about it.
For your next whistle, plan it such that the windway is not so high. When I’m at this point in developing a new whistle I reduce the ID of the mouthpiece a bit and turn the OD of the whistle body to match for the next one.
Carey beat me to the suggestion I was going to make. What type of tubing did you use? In the Low Tech whistle design the wall thickness of the tube determines the windway height. Guido uses a thin walled tubing which keeps the height to a workable dimension. Much of the common plumbing pipe available in home centers around the US has a thicker wall which can lead to breathy whistles.
There are other factors which could be contributing as well. The finished edge shape of the plug and its relationship to the edge of the blade can effect it as well. A voicing window that is too deep can also be a culprit (meaning the dimension from the plug to the blade).
Just some thoughts.
Feadoggie