A: Before you open up any walls, you need to check out few things. In my book Ed Del Grande's House Call I stress that most noises in a plumbing system can be traced in part to high water pressure. You need to make sure your water pressure is under 80 psi to meet most local codes.

My gut feeling on this one is that you have a very slow leak in your toilet flapper. This is why the noise happens late at night, when the toilet sits unused. Water may be leaking by the flapper until the tank water level drops to a point when the ballcock float finally has to open a bit to let in more water.

That tiny opening of the fill valve along with high water pressure behind it can cause an unbelievable roar from the plumbing system. I have heard it myself many times. So, check your flapper first before you cause an even bigger flap in your home by opening up the walls for no reason!