
Chris Plastow
If one can temporally block off the outside vent and simultaneously place a tissue paper to the fan inlet in the kitchen, two things can happen, 1) the possum falls to one floor indicating there are no leaks in the ducting pipe or the fan will hold the tissue , indicating that the ducting is leaking. This is a very unscientific test but it can give you am indication of what's going on. Alternatively, one day you cooked something that brought your fan to the neighbour's attention and from then on they were sensortised to it ! Good luck.