iPhone 6Plus vibrates when plugged in to power charger

Hello,

I recently got an iPhone 6Plus, which vibrates rather than makes any sound when I plug it in to charge it. My previous iPhone (an iPhone 5) made a sound when I plugged it in, as does my iPad Mini 4. When all three of these devices were set up I don't remember being asked about making a choice as to whether to 'vibrate or make a noise or both or neither, when connecting to power'. I can't remember whether my My Mothers' old iPhone 3GS made, or her if current iPhone 5 makes a sound or vibrates when plugged in, or if it does BOTH or neither. Her iPad Mini 2 only makes a sound when plugged in SOMETIMES. It may also vibrate at the same time SOMETIMES. I can't find any setting to change my iPhone 6Plus from 'vibrate when connected to power' to 'make a sound when connected to power'. I have also looked for the same on my Mothers iPad Mini, and my iPad Mini AND her iPhone 5, but without success.

I have noticed there are a heck of a lot of choices of 'ring tones' and 'alert' tones, and choices of 'vibration patterns' (including 'custom') as well as which 'alerts' to make decisions about, but I see NO option for 'connecting to power'.

Does ANYONE here know the answer?

Also, if the 'sound' made instead of vibrate can be selected from the same list as ring tones and text alerts?