It's a design issue. The heads are very sensitive to interference and must be grounded via a cable shield. In many boombox designs, the ground cable goes directly to ground. On others (like the Superscopes and some Sony's have this problem, very very common) the shield wire is not grounded immediately, rather goes through the Play-Record switch. If the play-record switch develops oxidation, a bad connection develops which is the same as an ungrounded play-record head. When left ungrounded, the cables pick up stray EMI or RFI and is amplified by the circuitry causing the hum.