What is the noise source ?
Noise will pass through the seals between the movable frame and the static frame. The glass itself will act as a diaphragm and retransmit the noise from outside to in.
you can try some painters tape on the seal edge to see if that cuts down on sound leakage. For the retransmission, your idea of outside or inside dense plants could help. Heavy curtains, if allowed, can help some..