I have an S90ES - plugged into a Lexicon Alpha interface using the mono output.  The keyboard doesn’t even need to be turned on and it creates a loud HUM or scratching buzz in my monitors or headphones.  It comes through in the recording tracks.  I’ve tried the headphone jack and running it to the interface, and it still has a buzz. 

I noticed once that when I lay my hand on the keyboard, it actually changes the pitch of the hum (turned off!).

There is no hum when I unplug the interface and play with headphones.  There is also no hum from the interface using other instruments. 

PLEASE help! There’s something I’m missing here.

Welcome to the forums.

You have a ground issue. If you are not familiar with electrical issues, you should consult a qualified electrician in you area. Without any information about where in the world you live or any information about your particular electrical wiring situation, it would be unwise to give you advice. The fact that you touching the gear changes the nature of the sound, is cause enough for me to tell you to consult a qualified electrician who can evaluate your situation. Safety first.