Hi,
Funny thing I started a thread regarding this yesterday and I think I have an explanation.
The reason it only happens from the volume buttons side is because the front assembly kinda gives in the metal backplate next to the volume buttons, under a light source, if pressed. You have to like be extra focused to see this, but it gives in by like 0.05mm probably.
Being the OCD person I am, I went to 3 different stores and tested out all the demos. Not to my surprise, every single one of them exhibited the same behaviour, exact same spot. The reason behind this has got to do with the way the phone is designed. If you look at any tear down of the 6 you will see some small support clip things, placed all around the perimeter of the screen. However, there is none near the area of the volume buttons due to there not being any room for them there. Probably that's a structural weak point of the phone. They're all that way. See photo here
https://d3nevzfk7ii3be.cloudfront.ne...OVGlFhcQS.huge
I cannot say I'm mad, maybe at best mildly annoyed at Apple. The volume buttons area has always exhibited some weird behaviour. My 3G and my 3GS phones all had light bleeding next to the buttons (remember the black tape fix?) and my 4S had some of it to a very big degree and now this. I understand there's physically no space there to put the support thingie but come on.
I don't know why yours makes the noise up there, people probably are checking this spot when the spot with no support is right next to the volume buttons. You need a better than adequate light source to see this but it can't NOT happen to some of you. The iPhone 6's structure simply doesn't have a support next to the volume buttons because there's no space for it, physically.
One thing is for sure, there's not a single phone with a perfect build one might have in their mind, that's not loose buttons, not a loose screen. It's true, the quality has decreased a lot but I think we're overreacting.