What solutions to remedy the noise in stainless steel radiators, during the rise in temperature... "rattling" for about 15 seconds, in 2 times. Boiler with natural gas condensation, temperature set between 28 and 40 degrees... Copper pipes in the ground.
The sounds heard are most often dilation noises when a climb in temperature, very difficult to eliminate. Check, however, if the copper pipes are not bridging the radiators, which may be the case, since you seem to indicate that they are "in the ground"... Sometimes, slightly loosening the pipe clamps enough to solve the problem. Another hypothesis: Boiler pressure too high (It must not exceed 2 bars, it is necessary to intervene on the boiler, and it may be that the radiators need to be purged.) This is not a very difficult operation.
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