Buzzing reverb sound when keys are pressed

Hi there

I have a Sansui Electronic keyboard, with a problem that appears and disappears:

There is a buzzing electric echo sound when the keys are pressed.

Sometimes the level of the buzzing is not so loud and we can still play, while other times it's louder than the actual notes and horrible to the ears...

it sounds only when a key is pressed (any key), and then slowly wanes .

Any ideas how I might be able to fix this? or which component would be at fault here?