Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
If one hears a sound inside his stomach but nothing comes out of him that would break his wudu, and he does not note anything that would indicate that, such as hearing a sound or smelling an odour, then he should not pay any attention to it and his wudu is still valid.
In responding to your question, Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states:
Prophet Muhammad (peace be upon him) explained to us the guidelines concerning hearing sounds inside the stomach and the state of purity:
“If any one of you feels something in his stomach, and he is not sure whether anything came out of him or not, let him not leave the mosque unless he hears a sound or detects an odour.” (Muslim)
The Prophet was told about a man who imagined that he felt something whilst he was praying. He said, “Let him not stop [praying] unless he hears a sound or detects an odour.” (Agreed upon)
Imam An-Nawawi said in Sharh Muslim, 4/49:
“This hadith forms one of the basic principles of Islam and an important principle of fiqh. Things are judged to remain as they are until there is certainty that a change has occurred, and having doubts does not alter the fact.”
It is not considered to be impurity until it emerges. Sounds may come from a person’s stomach as the result of food being moved (along the digestive tract).
Therefore, if one hears a sound inside his stomach but nothing comes out of him that would break his wudu, and he does not note anything that would indicate that, such as hearing a sound or smelling an odour, then he should not pay any attention to it. This is because the basic principle is that he remains pure. So, his wudu is not broken and he should not stop praying unless he is certain that something has come out of him. The principle is that what is certain is not overruled by what is doubtful.
Almighty Allah knows best.
Source: http://www.islamqa.info.