I have just turned on my Hot water heating sysyem and every so often it sounds as if someone is banging on the pipes with a hammer. (This also was occuring last year at the end of the season.)When I observe the pipes just above the boiler they also viabrate. Is this because there is air in the lines somewhere???? If so how do I get it out? Also I noticed three drain valves. One located by each of the two pumps and one at the bottom of the holding tank. Should these be periodically opened and should the tank be drained to remove sediment. Also are there any good sites that describe fully how this system operates and how to maintain it???