Here kitty, we had the same problem years ago. Is it a new house? We discovered that the chimney flues were fitted incorrectly & the lining/filing around them was not done correctly. Unfortunately we had to get it redone from fireplace to chimney stack. We had an engineer test it & it was quite a job. I hope that your problem is not as severe as ours was. Initially to see what the problem was they opened up the chimney Brest, it is something you need to get checked out though because it doesn't get better on its own - would your house insurance/ house guarantee cover this?