Buzzing noise when not playing - Marshall Amplification Marshall MG50FX 50 Watt Combo Guitar Amplifier with FX and MP3 Input

All amplifiers make a little bit of noise (hiss) but it is normally slight and drowned out by playing. However, assuming that this is not normal system hiss, here are a few simple steps to try and locate the source of the problem by process of elimination.

1. SAFETY FIRST - a common cause of a humming or buzzing sound can be a lack of earth (grounding) on the mains supply. With a 240volt AC supply this hum will be at 50hz (in the UK) - frequency may vary in other countries but should be similar. Check the earth first because of risk of electric shock. Until you are satisfied that this is not the cause of the problem DO NOT touch any metal parts of the amp. Remove the mains lead and try a different one. Try plugging the amp into a different mains outlet socket (preferably one on a completely different circuit).
2. Turn off other electrical equipment in the vicinity that may be causing interference. Move the amp a bit, try rotating it by 90 degrees to change relationship to things like mains electric cables in the floor.
3. If you are using the line-out socket to give a feed to a mixer, slave amp or p.a. system unplug this. Sometimes double earthing (through the mains lead of your amp AND the mains lead of the MIXER / PA) will cause an 'earth loop' due to a different resistance at each end. Putting a DI box with an 'earth lift' in between may eliminate this problem. Alternatively there are little gadgets you can buy to do this (I found one intended for car audio systems on Amazon that works very well)
4. Unplug any instrument leads - to eliminate possibilities of interference affecting cables, effects pedals or guitar pick-ups. Also unplug the foot-switch.
5. If that stops it plug a lead in without a guitar on the other end - does the noise return? If yes change the lead. If no plug the guitar in.
6. Try moving the guitar in relation to the amp (turn it around 90 or 180 degrees) - this will be pick-up to amp power supply interference.
7. Does the sound get better or worse if you touch metal parts of the guitar with your hand? This may be a screening issue within the lead or the guitar itself.
8. Try turning all the individual channel tone and gain controls to zero, turn off any effects on the amp panel. If that eliminates it only turn up the controls on channels that you are actually using.