AC Charging sound when phone is in my pocket

This is a weird one that just started happening in the past few days

I updated my phone to 4.1.2 about 6 weeks ago (UK T-Mobile Galaxy Note )
Once it finished i did a hard reset.

But for the last week or so ive noticed that when my phone is in my pocket i occasionally hear that "two-tone" sound that the phone makes when you plug in an AC Adapter.
If I pull my phone out too look at it, it is sitting at the Lock Screen.
Its not like the power icon is flashing as if it thinks its charging or anything.

I cant make it happen again when the phone is in my hand, it only happens when in my jeans pocket.

Any ideas?

update:

its started doing it when lying on the table now
When the phone is off, the lock screen comes on and in the middle of the lock screen it flashes up the message Charging 100% , then goes away.
its doing this about once a minute