There are a few reasons for pinging steam radiators. Residential steam systems of the type you are describing are designed to run at or around 1 pound steam pressure. Some folks think that more is better, but higher steam pressures drive water up into the radiators to make noise. The steam radiator hand valves must be completely open or they trap water to make pinging. A radiator valve that blows steam is no longer doing its job and needs to be replaced. The radiator needs to be level or sloped unnoticeably toward the steam pipe end to let water back to the boiler below. The boiler water must be clean or it will throw water up the pipes to the radiators also.
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