How do I get rid of the loud banging noise, after flushing the toilet?

If you mean a banging noise in the walls: Banging noises in walls associated with running water are nearly always because the water pipes involved were not properly secured by the plumber back when the house was built. Water pipes need to be anchored down firmly against the studs and other substructure when a house is built. If they're not... if there are long spans of unsecured pipe just sort of hanging loose... then the torque of the water flowing through them, and then suddenly stopping when the flow valve on the toilet closes, will cause the pipe to physically move from side to side, knocking against whatever wooden substructure is nearest it. Someone needs to locate the point in the wall where the pipe is actually making contact with a stud or whatever it is, cut open the wall, secure the pipe to the substructure, then close-up the wall and mud it, seal it and paint it. If you're lucky, the place where the banging is occurring is in the basement, where (assuming your basement is unfinished) the pipes are exposed and you can get at them easily. It will be just your luck, however, that the banging is happening inside a finished wall, upstairs, where tearing into the drywall can't be avoided in order to fix it.

If the banging is right in the toilet: It could be that the tank is not properly secured to the bowl (though if that were the case, there would probably be water dripping from the tank bottom onto the floor behind the bowl); or it could be that the bowl is not firmly secured to the floor; or it could be that the drain pipe beneath the floor is not properly secured to substructure (similar to the description in the above paragraph); or it could be that something in the water control mechanism inside the tank is malfunctioning. If it's inside the tank, that's actuallly the easiest and cheapest to fix. All of the components inside the toilet tank can be purchased, as a complete set, from any Home Depot or Lowe's (or equivalent) for under ten dollars; and installing them isn't rocket science. Rather than trying to troubleshoot in-tank problems, it sometimes makes more sense to just replace everything. If the noise is coming from inside the tank, replacing everything will undoubtedly fix it... even if, after it's done, you never really figure out what it ever was. If the problem is one of the other things I mentioned (eg, tank or bowl or drain pipe not being properly secured), then those things may or may not necessarily be big jobs... it just depends on which one it is, and why it's happening. If there's water damage to floors or walls, it may not be as simple as just tightening everything back down again.

In either case, if you don't know how to fix whatever's wrong yourself, there are probably handymen listed in the phone book who can help you. Just make sure he's licensed and bonded and insured; and make sure that he gives you a written estimate and then sticks to it. And it wouldn't hurt to stand there and watch him while he works so he won't pull a fast one.

It is probably due to water hammer. Try to lower the pressure of the water.

Excellent reply by DesElms but I think Shadow nailed it. This sounds like water hammer to me. But to be sure, does the noise start when you flush or when the tank fills and shuts off? Regards, Tom

If it is water hammer, I think it is, you could try a different ballcock that would close more slowly or you could add an arrestor. http://www.siouxchief.com/B_WaterHammer_Faq.cfm