Wow. I was literally about to create a new post about this. I'm feeling the same exact sensation when I go to plug in my 7 in a wooden stand, along side my Apple Watch. I just reached over to grab the phone lightly, and felt the most subtle vibration. I then took my finger and rubbed the back of the phone, and felt the same exact subtle vibrations you mentioned. It's so bizarre! For reference, I grabbed my 6s, and was able to reproduce the effect, although it seems to be much more dramatic on the 7. I found this gem of an article on the Apple Support Community which explains what's happening here in detail:
https://discussions.apple.com/thread/1022258?start=0&tstart=0
You can reproduce this very easily if you plug your phone in and ONLY hold it by the very tip of the lightning port with your thumb and index finger. Then, take your other hand and very gently rub the backside of the phone up and down. You'll feel the most bizarre subtle vibration, almost as if the electricity flowing through your body is turning the haptic feedback engine on. Also, if you put the tip of your ear up to the back of your phone as you do this, you can quite literally hear the electric current. What's strange, is that I never once noticed this with my 6s or any other Apple device. I only noticed it tonight, because I just got the 7 today and don't yet have a case for it. You can feel the vibration just by touching the back of the phone and sliding your finger up, while it's charging. At first I thought maybe I was getting interference from my Apple Watch which sits next to the iPhone dock. But from the article linked above, sound a like this is totally normal. Just REALLY bizarre!
[doublepost=1476162515][/doublepost]I'm really surprised more people haven't noticed this. I just grabbed my 6s again, and it's barely noticeable at all...yet, EXTREMELY noticeable with the 7. It's honestly almost unnerving...