fart exhaust!?

I recently had my muffler removed and installed straight pipes . Sounds great and deepest these fart sounds occasionally come at low rpm speeds , or when gear changes at high rpm.

It's not even a cool fart !! it sounds low and nice when revving down or braking. but sometimes it sounds very very stupid ?! any solution?

is it related to pipe shape?

2.4L 4 cylinder jeep compass


http://s1.postimg.org/9vhghu3a7/image.jpg