It's about 6 weeks old. What a beauty! House has never been so uniformly comfortable. And so quiet! Except for one thing -- it makes a whining tone in the middle speeds. Isn't there at high power when coming out of night set back. Isn't there at the lowest 3 or 4 speeds. Just in the middle where it spends most of its time.
Any idea what could be causing it and how to fix? If I run just the fan, that is right in the sweet spot for the whining noise.
For what it's worth, the whine used to be at the slowest speeds until the electronic air cleaner was replaced by a pleated passive unit. I'm guessing air flow must have something to do with this noise.