You most likely either have a problem with your monovalve or the air-return tube behind the glovebox. First thing to do is remove your glovebox and inspect the foam tube that connects to the air return on the dashboard. Most likely its crumbling apart and needs to be replaced. I used 1/2" pipe insulation from Home Depot. This is a cheap and easy fix.......
BUT, you could also have a problem with the monovalve. Order a monovalve repair kit for around $25 and throw the old one away. Obviously, do the cheap/easy fix first and then move on to the more complicated possibilities. If you need more help, check the archives, this is a VERY common problem.
'84 300D, 169k