Sounds like a respiratory infection. When in the early stages, they may wheeze occasionally. If it is a R.I., it will get worse. Respiratory infections are often fatal in tropical Boids, when they go too far. You'd better treat it right away. Start out by giving the snake a warm temperature, constantly. High 80s, to low 90s but with an area slightly cooler, also. The snake's water should be at this temperature, also. Humidity from 60% to 75%, measured just above floor level. Floor heat is best for this. Eliminate all stress, including unnecessary handling. A visit to a vet is the way to go. When transporting the snake, away from heat, put it under your shirt, for heat. Some Anacondas whistle through their nostrils before, and during shed. This doesn't sound like a wheeze. Look for excess saliva, and/or moisture coming from the nostrils, and mouth.
I have seen a few snakes that would hiss softly when agitated. I sure wouldn't count on it being that.
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To Buddhist hot dog vendor. "Make me one with everything."