A duct for fluid under pressure for an air-conditioning circuit having a conduit including a male end fitting of nominal inside diameter D0 and a female end fitting connected to it in a fluid-tight manner in or in the immediate vicinity of an enlarged axial portion of inside diameter D2 of the male or the female end fitting, the female end fitting having a nominal inside diameter D0′, and a tubular noise-reducing device that is mounted inside the male end fitting and defines between an axially internal end and an axially external end of this device at least one fluid flow channel. The enlarged portion of the male or female end fitting has a length L2 such that L2≧0.6D, where D designates their common value if D0 and D0′ are equal or whichever is the lower of the values D0 and D0′ if they are not equal.