Fizzy or buzz sound in my set up....can you help identify it?

I just have my keyboard hooked up to my AV Amplifier via the AUX input using an unbalanced TRS jack. When I turn it on and begin to press the keys there is a fizzy or buzz sound off some of them and its more noticeable as I move up the octaves... C6 onwards. It also happens if I press more than 4 keys that are next to each other. Its NOT a faulty audio adapter or the 50/60hz hum from the mains. Just curious as to what might be causing it... Cheers