I'm new to this board and to the DIY scene, so I apologize in advance if I leave out anything obvious...

We have been in this house for a little over a year. The house itself was built in 1982 or 1983 (at the moment I cannot remember exactly which one). The heat pump unit outside the house is a RUUD Achiever 9, according to the label on the top, and the blower unit inside the basement looks to be a GE. The model number printed in the metal label is BWV036A100A1. We knew when we moved in that we would need to replace it sooner than later, but we have been waiting to see if it will die while our home warranty is still active.

About two weeks ago I began hearing a noise when the heat was running. When putting my ear to any of the floor vents, however, I was unable to hear the sound. After some checking, I found that the one place where the sound was audible was the return vent on the top floor of the house. The sound itself is a moderately high-pitched wah-wah sound. It pulses rhythmically for as long as the heat is running - at a rate of perhaps twice a second. I am not able to hear it from the return on the main level of the house, or even if I stand near the blower unit in the basement (and I find that part interesting).

Given that this is a brand new sound, I am a little concerned. The question is, how concerned should I be? What might be causing this sound, and what might the best solution be?

Thanks in advance.