I have 0 clue what is going on with my laptop, and i really can't describe the sound so I uploaded a video on youtube. Hopefully it helps better understand whats happening. My laptop is running like normal and the noise is the only I noticed thats wrong with it.
I have the exact problem with my HP laptop fan. You need to first try to vacuum the holes the best you can and then use some compressed air to blow out dust. If this or other simple solutions is not working then you need to dissasemble your laptop and locate the fan. Search for the same model and see if it is hard to dissasemble. If it is hard, (like my model) then you need to send it in for cleaning or replacement of the fan. But try the simple methods and dissasembe it. I have the same problem and sometimes even louder and wierder sound but I can't dissasemble it because my model is too hard.