Hey, there. I created an account here to specifically comment on this thread, hopefully helping someone else with the issue. Green69 is absolutely correct in my case. I had Simonton VantagePointe 6500 windows installed and the howling was so loud that we could not sleep. It sounded like dogs/coyotes howling through the night. After finding this forum, I saw Green69's post and investigated. Sure enough, the holes in the side channel casing were causing the issue! Here's a pic of what to look for. Just caulk it up and you're good to go.