。
Wait until the kettle starts hissing.
Literal
Kettle [subj-が] shunshun say-until wait-[command-なさい].
シュンシュン is a 擬音語 for the continuous hissing/sputtering sound of a boiling kettle. Note the construction [mimetic] + いう — いう ('say') is the standard verb for 'make a sound' when the subject of the sound is described by a mimetic word: チンチンいう, ガタガタいう, etc. まで here marks the endpoint of waiting.