it could be feedback, what does ur amp do with ur guitar plugged in distortion on and the volume knob on the guitar turned right down? if there is no squeel its likely to be feedback which u can prevent by turning ur guitar away from the amp or puttin ur hand on the strings.

if it is feedback then yes all amps do it

youre probably not hearing things

youre amp is probably not broken, but u could allways take it back to where u bought it and ask them to have a look

i have a 15 watt marshall i like it alot and i think its a good amp, iv never had a problem with mine however they are the cheap marshalls, the expensive ones are brilliant amps