I had this problem 3 times already and each time a small magnetic particle had ended up on the bass speaker. The vent openings are big enough for small particles to enter the inside of your Macbook Pro. The easy fix is to open your back cover and wipe the speaker clean with a soft brush and while you're at it give the rest of the inside a good clean with the brush as well. With later Macbook pros be careful when opening the back though as there are 2 clips and 2 hinges and when you have popped open the front and side of the lid you need to pull it forward to unhinge the back lid. If unsure just watch on youtube how to open it. Apple has changed the lid design for quite a few years now.