I really hope this post helps everyone that has had this issue. I too had service out and they could not pin point and fix the whistling sound. But my husband easily found the problem and resolved it in a couple minutes with some duct tape, of course! The unit in the attic (or wherever you had it installed) that is the big grey thing that has all of the venting coming from it has 2 panels. What he found is that the air whistles between these panels and creates that annoying whistling sound. He took a long piece of duct tape and taped between the panels,. It's finally silent!! That sound was driving me batty so I wanted to share with everyone else who is experiencing this. Hope this resolves your issue! It did mine and I am so happy and it was so easy!