My red eared slider is making a quiet 'quacking' noise...anyone know why or if it's normal?

This is the second time I've noticed it. Both times it has been while basking under the heat lamp. It's not open-mouth breathing or wheezing. It's just making this quiet 'quacking' noise and every once in awhile a snoring sound...This is the first time I've had red eared sliders and don't know if this is normal or something to worry about. I called the pet store we got them from and the man said they don't make noise and he didn't know why it would be doing this...anyone have any idea?