The reason for the whistling is the peculiarity of the construction of the landing filter and the filter itself. The filter is slightly smaller than the gap and the sides of the filter remain. Over time, when the filter starts to get a little amused, the air is looking for the least resistance and starts passing through the cracks, past the filter.
You can solve the problem by plating the perimeters by isolating material. It could be a powder or a strip of noise insulation. It is difficult and meaningless to insulate the isolation on the filter box. After that, the whistle's gone.