Suddenly popping, crackling sound.

I got this Macbook as a leftover from my dad. About one-and-half year has passed.

And now I found my speaker popping some weird sounds. Or you can say, it's the frequently asked crackling sound.

This happens several minutes after I turned on my Macbook. But after I left my Macbook for about 10 - 20 minutes, it doesn't happen again. The crackling sound disappeared for a time.

What do you think about this phenomenon?

Should I ignore it? Or should I go to the Apple Store?