As long as it isn't happening only during playback of MP3 music, bring it up to an Apple Authorized Service Provider.
Issues from static electricity, to small components in the fans, to circuits not being manages properly by power management can all be to blame. If the issue only happens when the headphone jack is not connected, then it probably is a problem with the built-in speakers. If it only happens when you are using headphones via the minijack, then there might be dust in the audio jack
MP3 music that is compressed too much may have such audio anomalies. I recommend using a minimum of 192 kbps compression.