K4004B english

I use a K4004B in bridge mode (mono) for a subwoofer when you put your ear against the hp means a slight sizzle
is this normal? how to fix? (by adding one or more decoupling capacitors for example) thank you in advance

Hard to tell if this is normal or not, without being able to measure the exact freq. and level.
Is it 50/100Hz hum or higher freq. noise ?