A difficult one, Hannah...modern boilers use a force draught fan to keep the exhaust gases moving, and its this you can hear. Depending on if your neighbour has a balanced flue or not, it could be possible to extend the chimney upwards, but it really depends on how willing she is, I guess. Approach the problem as kindly as you can and say its making you ill...hopefully she'll see reason. Good luck.
John, January 2008