I suspect that the liner in your chimney vent has corroded or developed a crack. You should have it looked at by a chimney expert. (When was the last time you had it cleaned?) The probable solution will be to install a new liner. I would get 2 or 3 opinions on this from contractors you independently find out about to make sure they are honest and competent. You don't want to spend a lot of money only to find you're back to square one.