Low vibration when plugged into a 220v outlet

That sometimes occurs when not connected via a grounded (earthed) plug.

"Has anyone ever experienced a low vibration coming from their macbook pro when plugged into/ charging from a 220v source?"

The "vibration" is caused by the fact that the power adapter places one-half of the line voltage onto the computer chassis. Although annoying, the voltage is at a low current, and is not unsafe. You can alleviate the condition as an earlier posted noted. Use the 3-prong AC input cable on the power adapter, and plug the AC cord into a properly grounded AC outlet.

I noticed that, if my MB is charging whilst plugged into a 4-way adaptor plug (along with other things), such as a lamp, my mobile phone and a heated blanket, this causes the mild vibration on my MB case. I then discovered that unplugging the heated blanket from the 4-way adaptor causes the vibration to instantly stop. Leaving the phone charging and the lamp turned on appears to have no effect on the vibration at all.

So, if you're charging/operating other things/devices from the same adaptor (power source) as you're charging your MB from, then try unplugging these and just leaving the MB to charge form this power source on its own... see if that makes any difference.

It stopped the vibration for me anyway.