The repeated word "noon" works like the sound its title names. Four lines of the same word read as a steady, droning note, and they also feel like time refusing to move. It's the middle of the day, the sun is at its highest, and nothing changes. The sameness is almost hypnotic, and a little unsettling.
Then the poem turns inward. "A new window into my head" suggests the speaker has found a fresh way of seeing their own mind, perhaps through that endless sound or stillness. But the next line undercuts any sense of discovery: "a new sickness." The window doesn't bring clarity so much as illness, or an awareness of something wrong. We can't tell for sure whether this is physical, mental, or just a feeling of dread, and the poem doesn't say.
The mood is flat, tense, and claustrophobic. With so few words, the silence around them does a lot of the work.