A wind excited stretched string oscillator? This is known as an Aeolian Harp after Aeolius the Greek god of the wind. For pleasing people, multiple strings are frequently used, typically each a note in a pleasing chord. Great alternative to or addition to a wind chime. Never heard of using something like that to sscare birds. Long strings would likely produce low notes or just a rushing noise.
Muzzle loader? Hmmm, I have a .75 cal muzzle loading flintlock pirate pistol thingy (hand cannon) but the rate of fire is too slow and it is too labor intensive stoking it. There however, are special bird frightening shotgun shells that launch an exploding projectile. Essentially no shrapnel or damage hazzard if fired mostly upward (you might want to be a bit more direst, so to speak) and if they don't work on the geese, it is the 4th of July holiday season.
I would be looking to cobble something up using infrared motion detectors to trigger something geese wouldn't like. Unfortunately the setup I built probably would be a bad goose deterent as I used it to turn on a dishwasher water control valve to turn the water on to a rainbird sprinkler to keep dogs from leaving "Love Notes" in my landscaping (previous location was urban). Worked great on strays and great fun to watch folks with a leashed dog try to "Do it" in my yard. Picture someone with an eighty pound dog on a leash. They stand patiently on the sidewalk waiting for Bowser to do his thing. Bowser steps off the sidewalk, gets detected, water is turned on, while the air is still being purged (hissing sound) from the hose, the dog tries to jump a block away dragging the owner with a wrenched arm SOMEWHERE ELSE. A side benefit was that in two to three days it trained the Postman not to take a shortcut through my landscaping.
Geese could probably be detered with ultrasonics but it could annoy any dogs that heard it. Switching the ultrasonic projectors on with a motion detector wold mionimize the annoyance to other animals. I bought a few units from Sharper Image and some other catalog house to replace the rainbird gadget. Seems that during a drought when lawn watering to excess(running in street) was a punishable offence, some neighbor kids discovered but misinterpreted and misused my "system". Probably chased a ball into the yard and got sprinkled or something. They came back with their swimming suits and jumped back and forth over the sprinkler. They played so long (in our abscence) that water was running in the street. Didn't get a ticket but due to that concern, replaced sprinkler thingy with ultrasonics. Actually moved it to back yard garden area to keep cats from using it as a sandbox.
You M A Y try this at home kids B U T put the switch on the motion detector in the test position. This lets it work in the daylight A N D turns it off about 10 seconds after no motion is detected. Otherwise it will spray for some number of minutes (usually more than you want). This was a $8 motion detector with two light sockets. Just remove (or don't install the light sockets) and wire the 120 volt ac thingy to be controlled where the lights would have been.
Note: No waranty expressed or implied as to the fitness for any specific purpose of anything even remotely resembling what I described. Be careful, you could be killed or injured if you do something dumb. Not sure? Hire an electrician. Additional 10,000 word disclaimer available for those willing to pay to see it.
Patrick
