I was testing my discrete BJT audio amplifier yesterday. It does not have a speaker yet! The speaker is subsituted by a dummy resistor load. I heard sound coming from somewhere on the amplifier breadboard. The sound is very soft, but is at the correct frequencies as the function generator. How is it possible?

No capacitor, a few resistors, 12 BJTs, no coil or inductor.

 

It is possible that your dummy resistor may be of the wirewound type. Wire-wound resistors can produce a weak audible output when driven with a high current.

hgmjr

 

You could also be vibrating off a cracked resistor.