MacBook Pro (Unibody) Charger Hissing Sounds?

[rekhyt]

(Using my friend's Macbook)

My friend's MacBook Pro (Unibody) 15" has this problem where the charger makes this small hissing sound when the MacBook Pro is sleeping. (In Windows) It sounds like an oxygen tank slowly losing air or a small sound of the dialup sound. (No signs of problem though when not the MacBook Pro is in use/"awake".

What should I do? Will it be a fire hazard?

 

[tooz]

how loud is it? If you're really worried, a genius visit wouldn't hurt

 

[rekhyt]

how loud is it? If you're really worried, a genius visit wouldn't hurt

Sadly I'd have to buy a plane ticket to US if I wanted to visit a real Apple store/genius. It isn't really loud but is worrisome since it sounds like it's leaking air/a little hissing sound. (He bought the laptop from the online Apple store).

Are there any similar cases/what are the symptoms of a soon to be MacBook fire/explosion? (Which would be even worse for me since I'm using my MacBook on a wooden desk right now.