Once a year outlet buzzes

[homebody]

I've lived in my house a little over a year. Last year at exactly this time one night, an outlet started buzzing. Quite a loud buzz that lasted about 5 seconds. It repeated about 2 times in the same night. The outlet is located on an outside wall, and I live in a very cold climate. I didn't do anything about it as it never happened again.....until last night. Same sound. There was a space heater plugged into the outlet, which I promptly unplugged. The odd thing is that it buzzed again about 15 minutes later when nothing was plugged into it!
Since it only has happened at this time of year, I'm wondering if it is due to some kind of condensation occurring as it has been quite cold, but this week has warmed up about 40 degrees to reach into the upper 30's.
Should I be concerned? Any ideas on what might be causing this??

[Java_Bob]

could be moisture, pull the outlet out and take a look see

[homebody]

Hi--
We did take the face plate off and look at it last night. Seems to be dry. I know nothing about outlets, but I did notice what I assume is the ground wire is kind of touching the side of the outlet box all the way up to where it is connected. That wire is just bare copper. I thought maybe if that were to vibrate for some reason, it could make a noise vibrating against the metal box? Why would a ground wire vibrate?

[HayZee518]

only way a ground wire would vibrate IF is was conducting some sort of current. you might try turning off the circuit removing the outlet and tightening the screws that hold the wires. essentially there are no moving parts inside as the screws and the contact is one complete piece - both the neutral and the hot part.