About three weeks ago our 10 year furnace's blower motor (which also blows for the AC) died (the bearing seized). HVAC company comes out, removes the squirrel cage and motor, puts a new motor (with a different motor brand/model) on the same squirrel cage, and re-installs it. It's fine for a day or two, but now there's a noise when the fan kicks on.
Best way I can describe it is that it sounds like a loud electrical buzzing (almost like a buzzer on a dryer, even). The noise disappears after about 3 seconds, but occasionally if you stand near one of the return registers, the fan sounds makes a rhythmic tapping -very- much like a diesel engine on a truck (at idle), though it sounds very distant. The fan is not belt driven and the motor is attached to the squirrel cage using rubber washers.
After a run cycle, there is no excess heat and we use basic 90-day MERV 8 pleated filters (nothing fancy). The AC coil is a little dirty, but the fan is clean. Is this noise just the noise this particular motor makes or should I be concerned?