I have a goodman AEPT 060 5 ton variable speed air handler and heat pump heating and cooling my 4000 sq ft home in washington state. I had a 16" cold air return in central part of home and a seperate 10" return in master bedroom but noticed a lot of noise coming from my 16" cold air return. I decided to add an additional 12" return about 15' away from the existing 16" return in hopes this would quiet the system down. Now the noise is coming from the 12" return(whistling noise). The 12" return is basically a 12"x12" box that transitions to 12" round pipe. From the back of that is 25' of insulated flex duct going into my cold air return box on top of my air handler. Any ideas??