1. Does it do this when you move it to radically different locations in your house or studio fluorescent light ballasts in your place or next door (or even upstairs or downstairs will definitely do this. As will ground loops. But moving it around is a good way to check for RFI and ballast hum etc.

2. Does it do this when you listen via headphones straight out of the synth if not then it's nothing to do with the synth.

3. Does the noise vary or abate when you have it opened up Try moving some wires around etc  but please make sure you did (2) first.

there are other sources of problem but these should take care of 99% of the likeliest ones.