I have a two zone base board heating system, after the first few weeks of the heating season I hear in a few places in the house a bang when the thermostat reaches the set tempature. My oulet pipe from the furance has two tees for each zone and at the end has an expansion tank. Are the bangs related to heat expansion of the pipes, since the pipes are in the walls I can't brace them. What are my options?

Check all the places where pipes come through walls, floors or are in hangers. Many times the banging is from the pipe binding against something. When the pipe finally expands enough it overcomes the binding and then bangs to a new resting place. As the pipe cools, which takes longer, it comes back beyond the binding point so it can start the cycle over again. A tiny dab of high temperature grease in the right place sometimes cures it. Most times finding the bind is the hard part. Check when the pipe is at it's hottest.

It "sounds" like your right, No pun intended. Somewhere within the wall or where pipes are coming up through the floor is an opening that is to small for the pipe. Its rubbing against something. As the pipe is getting longer during the heat cycle it is binding, then once the pressure gets built up it gives causing the bang.
I would first check where the fins rest on the metal brackets in the baseboard cabinet. There should be little plastice guides that the fins rest on. Without them the expansion of the pipe in this area can cause an awful racket. then check where the pipes either enter the wall or go through the floor on the ends.
quite often then not the hole is to small or because the plumber did not center the pipe properly its hitting one side or the other.
If you find this is the case, you need to purchase one of those plastic roll up cutting boards for the kitchen. Its made of a very tough plastic. Cut it into strips and push it down between the pipe and the hole. It should help silence the noise. Ideally you should cut the hole larger, but its difficult without hitting the pipe.

Gentlemen, thanks for your response, I can't get to the spots where the pipes are banging unless I open walls and ceilings. I was hoping to have a solution where the valves are or could there be a problem with my expansion tank. Since the problem occurs after three weeks into the heating system, I think the build up of pressure becomes a problem?

More likely the scenario is that after three weeks into the heating season the natural moisture level in the wood has dried out making it worse. Add to that the possibility that the weather has gotten cooler and the furnce runs more often making it more noticable. My guess is that the expansion tank is operating fine. If it wasn't, the pressure relief on the boiler would have opened, discharging water onto the floor. If there is a valve in the line between the boiler and expansion tank, be sure it is completely open. There is a pressure regulator on the water feed to the boiler that maintains a contant pressure regardless of temperature, so it cannot build any additional pressure. The water feed pressure is set to match the elevations of the high point in your heating system. If water is circulating properly, which it appears to be doing, then don't change the pressure setting. Your problem is expansion of the pipe runs due to heat, not pressure. Sometimes installers forget that they are sizing pipe runs with cold pipe and don't allow enough clearance for the pipes in a hot expanded condition.

What is happening if this issue starts after some time into the heating season is the pipe is running through a hole cut into a piece of wood somewhere in the offending wall area. AS this area warms up the expansion causes the pipe to stick in the hole, thus as the pipe frees itself it pops and makes the noise. Nothing can be done anywhere other then where the noise is coming from. If this means cutting holes in the walls that would be the fix. If this is a fairly new home, I would be concerned as this is most likely a spot where the pipe can wear out. The result is a future leak. Do not wait for this to happen as the results can be a big mess.

As far as a valve or pressure, The pressure should remain around 12 PSI or so year round. If you find the pressure in the boiler goes up beyond 12 psi to over 20 or so when the boiler heats up and goes back down when it cools. Your expansion tank is water logged or you have a feed valve problem. Older pressure tanks that have a drain valve on the bottom must be emptied every few years or so as they become water logged and pressure issues become a problem. If you have a solid take without any holes other then where it connects and where air can be put in you have a pre-pressurized tank. If the pressure in the tank has changed so will the pressure on the boiler. This is fairly rare however. Regardless the pressure of the water is not causing the banging noise.

The noise happens when your thermostat is satisfied, meaning your recirc pump stops, meaning your check valves, if any, are slamming shut and this is being exaggerated by a build-up of air in the loop causing the noise to resonate in and around bracing in the walls. Make sure you have air vents(spiro vents)or air bleeder vents, preferrably on the high side of your boiler lines. If they are manual bleed valves, open them with a recirc pump running until the loop has made several passes. Then close the valves. Even if you can't hear or see air coming out, it doesn't mean it's not dispersed throughout the system. Sometimes cheap swing check valves are used. Spring checks are preferred because they close quietly. The above is just one more thing that could be adding to your noise problem. I've seen this on newly installed systems. Make sure you get the air out.

askaplumberfirst.com thanks for your advice, each room has a self bleeder valve but they never seem to do the job. If the cap is on to tight than air does not escape and if it is to losely than water comes out. The swing valve is an interesting option, what are the pros/cons of replacing with my current valve. What about adding a relief/pressure valve after the shutoff valve for each zone?

I have an older home - lived in it for 6 years with no pipe banging but occasionally a pipe froze when the temperature dipped to 10 below. My plumber put antifreeze in the system to address that ( during the summer) Next winter, my pipes banged to wake the dead. Plumber couldn't solve the problem after repeated attempts over 2 years with horrible noise the whole time. Got new plumber. New plumber drained the antifreeze. Problem solved. Apparently, antifreeze can cause an "inversion" kind of problem, where it floats on top and traps air. I love my new plumber.

Gary check out the web link I provided. It may answer many of your questions.
Self bleeding valves have a float inside them. As the air fills the little bottle shape device it allows the float to drop. On top of this float is a little needle valve that opens up and allows the air to escape. At it does this the water fills the bottle back up and the float comes up and shuts off the valve and the air is now out. If the valve leaks when you remove the cap, Some have self bleeding methods in which you can force the air out by bleeding the water so this may be normal. others do not so leakage may be a sign of defective valve. These are cheep enough to change so if you suspect its bad. change it. Just be sure to take the pressure off the boiler before doing so. A lot of water can come out of that little hole real quick.

http://www.heatinghelp.com/heating_howcome4.cfm

hheh this happened to me once and before I happened upon this wonderful board I found this ehow.com/how_117481_quiet-noisy-pipes.html , it's basic idiot proof how-to kinda thing I found helpful, maybe you will too. good luck -Fishfool @ The Reef Tank