We had a Lennox SL280v two-stage furnace installed for our first floor a few weeks ago, and we are very satisfied with the performance. However, when the unit runs you can hear a loud hum coming from the blower. The hum seems to change as the blower ramps up and down. I suppose its coming from the ecm motor. Our upstairs SL280v which was installed last summer has a very similar noise, except it is less noticeable. Any ideas?? Thanks.