Built-in speaker produce sound with much vibration

I have a MacBook (13-inch, Late 2009) that I use frequently. So, recently my built-in speakers started to make sound with much vibration in certain cases. This is most noticed when I switch on my mac: the startup sound cames with much vibration. While hearing music ou having a call, this problem is less noticed, but for a good ear, some vibration is noticed in some sounds.

What can this be? Thank you.