If you leave open the windows of a house on a windy day, it will generate strange, loud noises—kind of like when you blow over the lip of an empty bottle or jug. (In this case, though, Hughes is referencing the idea of an opera singer hitting a super-high note and making a glass shatter.) Again, this helps demonstrate the fragility of all things when confronted by the wind. We hear that
[...]. The house
Rang like some fine green goblet in the note That any second would shatter it (16-18)
In the same way that the landscape might flap up like a tent, and the magpie gets knocked out of the sky, this simile tells us that the house seems like it might shatter under the force of the wind.