iPhone 6 Plus Static Noise With Earphone

I've bought an iPhone 6 Plus nearly a month ago. Everthing seemed fine until one week ago. Since last week, I started to hear static noises through my earphone after a while (3 - 4 minute later after the earphone plugged in). This incident only happens when the earphone is plugged in and when I touch the screen. Most probably it started with 8.3 release, but I'm not sure whether this is a software issue or hardware issue. I gave the phone to an apple retail store, they installed the firmware again, but the problem continues. Does anybody experience this kind of problem? Should I go to the retail store again or just wait for abother update?