My DD gets that, but she's got all 8 front teeth, so when she doesn't get a good seal, some air gets in and she makes that sound.
Seems like a latching issue to me. When she gets a little bigger and gets better at sucking she might not have that problem. As long as she's swallowing and getting milk, I don't think you have to worry. But you can try different holds, like the above person mentioned.