Subject: What could cause a sulfur smell to come out of my air conditioning ducts

There is a strong intermittent sulfur smell that comes out of my air 
conditioning ducts when either the heat or air conditioning is on.  The 
heat pump guys say it must be something in the vents -- I'm skeptical of 
that since it doesn't happen all the time.  Any idea what it could be?

Re: What could cause a sulfur smell to come out of my air conditioning ducts