I have an intermittent problem with water hammer when I flush the toilet. I hear this jackhammer like sound and I can see the flexible supply pipe going to the toilet vibrate. There is no water hammer anywhere else. I live in an apartment style condo. If I take the lid off the tank and hold the float up it stops sometimes. I have been told 2 different answers. One is to replace the fill valve. The other thing is to replace the seal on the tap of the supply line, which is much harder to do because I would have to get the water shut off for the whole building to do this. So which is the most likely culprit?