Hamster wheel rattle

[The_Breeze]

Furnace fired up and I hear a hamster wheel. It smooths out. Should I be thinking bearings?

[MJC62]

Can you access the fan? Make sure nothing has “moved in” such as a mud robber nest etc. that would throw the fan out of balance. Doesn’t take much to throw a squirrel cage out of balance.

[Bob Denman]

Could it be some leftover construction debris that recently found a new home?

[The Gritz Carlton]

Had the same exact issue when new. Factory found a loose screw in the squirrel cage. It rattled when first started up but centrifugal force threw it to the outside and it became stationary and quiet. Post#3 is your answer...probably...I think.