Ali, Matilda made that same sound on her back & even had clear fluid dribbling out the side of her mouth which apparently was stomach acid. We often did the cloth nappy prop up on the side and kept the cot on an incline. The MCHN actually told us to put Matilda to sleep for day sleeps on her belly because that was the only way she would settle. She never once slept on her back until she was around 13 months.