Weird open sound

I have been searching the internet to find the answer to my question, but so far I have found nothing.

I'm having an issue with the sound my MacBook makes...not the start-up sound...but the sound when the laptop is opened or closed after it's already been on. The sounds it usually makes is deeper and slower.

My MacBook is approx. 2 years old and without problems until now.

Any idea why this would be happening?

That's your Superdrive checking for media. It's not unusual that it makes a sound or that it changes in tone or pitch.