Feedback, the dreaded "sound man's (and sound woman's)curse," is caused by a regeneration of sound leaving a speaker and entering a microphone. This tone - a sustained shriek - is a self-perpetuating cycle which can be stopped by decreasing the volume. Sometimes referred to as a standing wave.
Howl or whistle caused by the recirculation of acoustic output of an amplifying system. In a hearing aid the sound emanating from the receiver may reach (or feed back into) the microphone and be reamplified until it builds up into a high-pitched whistle.