Today I came to see why the filter is whistling. The filter was a merv 10. She. Is going to use merv 8 filters. She has a zoned system. The static pressure is high when only one zone is open. If the noise issue comes back then they will need to have a comfort specialist come and give estimate to releave pressure in the supply air when only one zone comes on. They do not have a hard start on the outdoor unit. Today I did a msc prepaid 29 point ac tune up. They bought 12 - 16x16x1 filters.