reply to lutful
Re: noisy furnace fan A few suggestions/ideas, it's possible that:
-The motor bearings are being to fail (new motor)
-The blower has become loose on the motor shaft and rattles (tighten bolt(s)) - applies to belted and direct drive systems.
-If you have AC and if the guy that installed the ductwork added a bypass damper for the AC coil, it could be flapping as the blower kicks in. (the damper should be open in winter, closed in summer)
-someone might have dropped something into the ducts (got kids?) which may have dropped down near the furnace. (duct hunting!)