I have 50 year old 24 unit building with force hot water heating. I have one unit that has a high pitched sound running through the pipes when the unit calls for heat. The zone valve is fine. I have tried reducing the water going into the pipes by partially closing the valve so not as much pressure is going into that unit. It is the farthest from the boiler. The boiler guys says it might be pressure going through the elbows of the pipes but after reading up on this could it be lime/deposit build up? When I drained some of the water it was very dirty (I drained it because I thought it might have air in the lines, but it didn't). Any thoughts