I've got a Yamaha P90. For sometime now I hear quite a loud (and irritating) static noise when I plug my headphones or speakers into my keyboard. Whether the volume is high or low the static noise stays the same. The only thing that reduces it or completely takes it away is if I plug headphones into each of the headphone outputs - so one pair that I use to for listening and the other pair to stop the static noise.

Has anyone else had the same problem? Or can someone tell me whether this is an internal problem or if something externally is causing it? I read somewhere that being near electrical signals or something can make it worse?!

Any advice would be much appreciated!