Bad air coming from vents

Ever since July 2015 we have an acrid
odor coming from our air vents..not just
with the AC as I first thought but with
our heat as well. Does anyone have a clue
as to what this could be?
The smell sometimes keeps me up at night
and I smelled it in my family room for the
first time yesterday.
I think my husband thinks I am imagining this
but I know I am not since it's been
going on now off and on since July.