When I had my system installed 2 years ago I mentioned a whistling sound on this forum and received a ton of great suggestions. One thing that was mentioned was that my return duct-work was insufficient. I procrastinated for 2 years [blush] but finally added another return vent and the whistling went away. I debated bumping my old post, but decided a new post might make it easier for someone else to find if they have the same issue. Thank you again to all who helped me with that and my other questions.

Glad to hear you solved the problem, and thanks for the follow-up. Somebody else may well benefit from your experience.