Depends where the "bonk" noise is coming from,it could be your stopcock,find the main stopcock in the house and hold it while someone flushes the loo,you may feel it knock when the loo finishes filling or hear the bonk noise more clearly in that area,if so turn water off and only open the tap enough so you have pressure but not fully on,the spindle in old stopcocks wear and causes it to rattle making this sound,the sudden stopping of the water flow makes it rattle.Don't know if this will cure it but you have nothing to loos(pun)by trying it,you can always re list your question.Good luck.
Rich
February 2010