I have 3 of these chargers and ironically it's the newest one that makes a similar noise to yours.
It started doing it about a week after it was 1yr old so I couldn't send it back
I took the charger apart and cleaned it (they get full of dust) and squirted some GT85 in the fan and that seemed to cure it but after a while the noise came back, at which point I discovered that if you blow on the fan the noise goes away
If you want to replace the fan just go on ebay and purchase any 6V 30 x 30 x 10mm fan, I bought a couple for about £5 from China just in case:
http://www.ebay.com/itm/1x-Brushless...item2a10f42192