Our aircraft is a 'straight' 690 with the Keith Freon kit installed.
Every time I switch on the factory air conditioning, a very loud noise can be heard. This happens when it is pressurized or not. However when pressurized, after reaching roughly 13000 ft (cabin set at 8000ft) the sound slowly fades and goes away eventually.
The previous owner told me that it could be a loose or damaged seal. It sounds like it is coming from the back of the cabin.
Has anyone ever a similar issue? I'm curious as to what the cause is and I don't really know where to look.