My car grunts

[Akiyo]

So not to long ago I noticed a weird grunt like noise coming from my Protege. When Im idling it will grunt a few times every once in a while and my RPMs will raise about 100 or so rpm. It doesnt do it under any load only idle.
Has this happened to anyone else here?

Edit: Its a 2001 Mazda Protege ES
heres a video so you can hear it for yourself:
http://www.youtube.com/watch?v=4dsmt...ature=youtu.be

Any help is appreciated