Studio monitor buzzing issue tech savys

I have a pair of KRK Rokit 5 monitors hooked up to FL studio via a 3.5m(sth like that) rca cable to to my laptop audio jack, and they make this annoying subtle fuzzy sine buzzing noise, they don't make any noise when the jack isn't hooked up to anything, they also make the exact same sound any time i plug the cable to anything such as my iPhone, anyone can troubleshoot based on this? I would really appreciate the help.

Is eBay trust worthy?

I want a iPhone 7 Plus for the low & i was wondering if I should get it off of there. Is it legit?

How do parents access their children's text messages on iphone?

The reason I ask, is because somehow my parents not only read my private text messages, but were able to get copies of particular ones sent to their phones. At first I thought it was a spyware app my Father somehow installed. So, I have since updated my IOS to a new version to hopefully have gotten rid of(or disabled)any spyware.