Yamaha keyboard PSR-S900 Crackling/Display & Lights Flickering

Hi there

I am currently having a problem with the Yamaha PSR-S900 keyboard whereby it is making a crackling noise and the buttons are flashing, screen is flickering etc.  It seems to make this crackle at the same level regardless of what I am doing on the keyboard.  The keyboard functions still just with this crackle and flickering lights/display. I've already replaced the power unit but it still has the same problem.

Would you have any idea on what the issue is and how best to solve it?  Many thanks.