If I could confirm that the noise was from a water leak in the serpentine. What I did was close the stopcock so you don't lose. Is there a way to solve even provisionally? With epoxy putty or something? Or else there is some greater risk of keeping it closed and just turning it on to shower for example .. just this problem finds me without money to call the plumber .. Thanks a million for your answer is helpful!
Unfortunately the solution is to replace the serpentine. Depending on where the loss is, some repair may or may not be made. If it is in the grill area or nearby areas (where it receives the flames) nothing can be done since these will burn the putty. As I said, the solution is the replacement of the serpentine and if the water heater is old for a matter of costs it is replaced.