It sounds like you haven't read my original post. I have 6 GHL Mitras lights also over the same tank. Even though there are 6 of them, and each has twice the wattage of the Kessil, I don't hear their fans because they have variable speed fans.
Anyway, I just opened up the Kessil to replace the fan. Unfortunately, this was another case of my being "ready-fire-aim" when taking on a project. The Noctua computer fan that I saw recommended as a Kessil 360 replacement fan comes in a housing. The newer A360X light has the fan mounted right inside the heat sink. No housing. This actually seems like a better design, but I can't use this fan as a replacement. Perhaps there is a housingless fan I could use, or I could try to pull a fan out of the housing, but I'm starting to run out of steam on this project.