Loud crackling noises from audio equipment are often caused by connection problems on the input side or within the controls. Check and clean all plug-in connector with spray contact cleaner…. More of this stuff later. Also move the power lead about a little as sometimes a poor connection here can result this type of noise.
Screw type attachment terminal can also loosen so check them.
If your system has a combined on/off switch and mechanical volume control, this problem could well be dirt in and/or degradation of the volume control pot(potentiometer; it’s a variable resistor with a fancy name). Over time dust can get between the control’s track and wiper contact. If this is the case you will need a can of spray contact cleaner: Radio Shack will have it. You will need to take the covers off the unit to get to the control.
I guess you know this bit but I’ll say it anyway…… disconnect the mains power first. Try to spray the cleaner into the control: there are often openings in the metal case.
The tone controls are also subject to the same problems and the same remedy.
Hope this helps..