PC Engine coregrafx possible problem

[Manhattan Sports Club]

I bought a PC Engine Coregrafx from eBay recently which came with a stereo AV cable, which I switched with my Genesis model 1 mono AV cable that has the same input. This is because there was some slightly noticable bar of noise going from down to up when using the stereo AV on the PC Engine, and the switch of cables reduced it almost entirely. I am wondering if this is normal? It's only barely visible when the screen is entirely black, and not really visible during gameplay. I am using a 3rd party AC, which says:

"AC/DC Adaptor
I/P/AC: 100-240VAC
50~60hz
O/P DC: 12VDC 1A"

In case that affects anything.

On another note, I don't get the same effects when using the stereo cable on the Genesis. Both are relatively cheap cables.