IPhone 5 vibrated for no reason while plugged to charger

Question

I have a small problem (?) with my 1 week old iPhone 5. I have noticed yesterday that when phone was plugged to the charger
it vibrated once with no reason. Vibration was the same kind like when you switch the phone to the silent mode. The phone was on a table and when
it vibrated, it did not turn screen on or show other notifications of any received or updated data. I have also checked all possible apps for any updates but no new messages in Whatsapp, facebook or any other app.
I have seen some topics where problem was that email was on automatic update and phone vibrated upon arrival of new mail but my automatic update of email has been turned off since the day 1 and in my case this is not the problem.
Phone was not on silent mode or connected to any other device by bluetooth or airplay.
When I connected it to the charger, it came up with normal connection sound. I left it on the desk charging and this vibration appeared suddenly after the phone was connected for some time 30-60mins and completely untouched.
Do you have any advice what else this might be?

Answer 1

it could be a software problem but it could as easy be a hardware defect
a restart would likely be 1 of 2 things
1 a software error that the ios operating system can't handle and it makes it restart to try and fix it rather then deleting everything in it's storage which in many ways would be a much worse way of trying to deal with an error
2. a hardware defect which result in 1 as the software try to do something which fail huge! because the hardware is broken not as much because the software is