Fire in the chimney flue can sound like low rumbling, roaring, or staccato popping.  Flames may shoot upward out of the chimney.  Others are slower burning and less obvious. 

First, call 911.  Second, do not extinguish the fire in the fire box.  Tossing a small cup of water onto the fire and closing the door can create a column of steam that can extinguish the fire in the chimney.  You can also close the dampers to reduce the airflow as much as possible.

Chimney fires can crack or damage chimneys or liners, which can cause a house to catch fire.  Always get a proper chimney sweep to clean and inspect your chimney after a fire, before using it again.