Static noise when I'm charging my macbook

Hi.

I have a macbook pro, and my charger's cord died yesterday. I don't have a lot of money, and genuine apple chargers are like 120 dollars a piece here, so I bought a cheap chinese knock off. I made sure that it had the same input/output voltage etc. and it works fine. The problem is, whenever I charge with it, and I use headphones, there's a constant static noise in the background.

I don't know why, but I'm guessing I bought something that isn't any good?

I read on the backside and it says:

1. This device may not cause harmful interference
2. This device must accept any interference received, including interference that may cause undesired operations

I don't know what the hell this means, but I'm guessing it's in connection to my problem. Anyone aware of how this power thing works that can inform me?

Thanks.