I have a similar problem with my 6. I'm on Sprint FWIW.
I get a buzz/hum/crackle about 5% of the time when I'm streaming audio on LTE. No noise like that when I'm on 3G.
This happens when I'm plugged into my car aux input. I'm not sure if it happens with headphones too.
When I am playing music stored on my phone, it doesn't seem to happen. I think it's happened once.
I've Googled this issue, and a few threads show up on other sites. There are even a few old threads on this site. Many seem to say that this is normal for LTE.
I'm not sure I buy it. It seems like there would be a lot more people asking about it on the internet if this happens to everyone's LTE phone.
Also, it seems to happen more when I am driving through a canyon or under a highway interchange (mix master, spaghetti bowl, etc.).
Is this a hardware issue?