My pleasure, but I'm very sorry to hear the symptoms sounds the same.
So, with Gingersnap it progressed very slowly and at first my vet thought she was just eating her food too quickly. The frequency of the noise increased, and I brought Gingersnap back to the vet a few months later. Then the vet could hear some faint noises but not see anything on the x-rays. They started Gingersnap on a month of antibiotics. That didn't help so they gave her a month of antifungals. That also didn't work so they gave her a more serious antifungal that isn't allowed for ducks. At this point the noise stopped increased in frequency, but didn't decrease in frequency. This is also when they started seeing a mass in her respiratory system. They weren't sure if she had aspergillosis or not, they suspected it as an option. They wanted to scope her, but we afraid to dislodge the potential fungal plug. I think we did the more serious antifungal for a few months until the noise started increasing again. The x-rays showed that the mass was much larger. We started nebulizing an antifungal medication for her 3 times daily and that helped her a lot. She was actually getting better, but then she began getting worse again. Eventually it got to where she was clearly miserable, breathing was very difficult, and we had her euthanized. The vet was surprised with how entirely full half of her chest was with aspergillosis. The diagnosis was only entirely confirmed with necropsy. I should point out that in the last 3 months of Gingersnap's life alone I spent about $8,000.
However! When my other duck, Pigweed, started making that noise I made an emergency appointment at the vet the next day. I asked that they give me itraconazole - the medication that worked to stop Gingersnap's noise from increasing. Pigweed made less of that noise within a week. I gave her the medication for 6 weeks only. A year later she's fine. The total necessary cost was about $300. I opted for extra blood testing to monitor organ function. The drug can be hard on the liver and I wanted to give it for as long as possible without hurting her. Obviously my ducks are pets.
My understanding is that aspergillosis can grow at different speeds. Sometimes medication just keeps it from growing, but doesn't shrink or kill it. It is REALLY hard to treat - but acting quickly is critical. I really hope your duck doesn't have this.