This problem occurs with transformer power supplies when the transformer core is badly packed and there is an air gap between the individual transformer laminations. In this case, tightening the core screw connection can provide a remedy. It is also possible that the strong magnetic field of the transformer causes the housing cover to vibrate. Especially if it is a sheet steel housing and the screw connections have loosened. Furthermore, the hum with the 50 Hz mains frequency also occurs when the load limits of the transformer are exceeded. If mains hum is heard during reception, the sieve chain of the power supply unit is no longer working properly; the charging and sieve capacitors could be incorrectly dimensioned or defective.