I have a Lennox G71 furnace, has just reached two years old. I run the fan on it full-time to keep air circulating, filtering, etc.
On initial install it was virtually silent, just as it is supposed to be. However, over the last couple of months, I have noticed the fan getting louder even though it is reporting the same CFM on the front panel as it always has. Some of the noise is a higher-pitched whine. If I "nudge" the furnace with my hip, the pitch changes.
Regular maintenance always performed. I thought these things were built to last and could handle me running the fan full-time. Maybe not?