During the practice, we must observe each and every mental and physical process which is arising at the moment. In the beginning of the practice, we must contemplate the abdominal movements as instructed by the Most Venerable Mahasi Sayadaw. Contemplation of the abdominal movements is in accordance with the Maha Satipatthana Sutta, the Discourse on the Four Foundations of Mindfulness. In that discourse, there is a chapter concerning mindfulness of the four elements. There the Buddha teaches us to be mindful of the four elements when they arise pathavi dhatu - earth element, apo dhatu - water element, tejo dhatu - fire element and vayo dhatu - wind element). Not only these four elements but all mental and physical phenomena must be observed.
We must understand that the earth element is not actually the earth. Instead it refers to the true nature of the earth element. Earth element is the name given to its individual characteristics, such as hardness and softness. The scriptures say. "Hardness and softness are the individual or specific characteristics of the earth element" - so when you thoroughly realise hardness or softness in any part of your body, it means that you are realizing the true nature or individual characteristic of the earth element(pavthavi dhatu).
Water element is not actually water but the term given to the individual characteristics of the element. Fluidity and cohesion are characteristics of the water element (apo dhatu). When you realise the nature of fluidity or cohesion in any part of your body, it means you are realizing the water element. Similarly, the fire element is not really fire, but the specific characteristic of the element. Heat and cold are the specific characteristics of the fire element (tejo dhatu). Wind element (vayo dhatu) likewise is not wind but the term given to the specific characteristics of the wind element, that is, movement, motion, vibration or support in any part of your body. When you feel, realise and rightly understand this movement, motion, vibration or supporting nature in any part of your body, it means that you are realizing the wind element. This is mindfulness of the four elements.
The Omniscient Buddha said, "Any mental or physical process must be observed as it is." When we sit in any comfortable position and focus our mind on the mental and physical processes, we may not know which object must be observed first. So, to overcome this difficulty, the Most Venerable Mahasi Sayadaw instructed his meditators to begin with the abdominal movements. When we breathe in, the abdomen rises, when we breathe out, the abdomen falls. We should focus our mind on the abdominal movement. When the abdomen rises, we should note it as rising, and when it falls, as falling. In this way: rising, falling, rising, falling. Thus we can feel the inward and outward movement of the abdomen. This specific characteristic of vayo dhatu must be thoroughly realized by meditators so that they can destroy the false view of a person, a being or a soul. They must observe the inward movement and outward movement of the abdomen or the rising and falling movement of the abdomen, making a mental note of rising, falling, rising, falling.
During the contemplation of your abdominal movement, when you hear a sound which is loud enough to be noted, then you should note, hearing, hearing, hearing. At the beginning of the practice, you may not overcome it, so you should note, hearing, hearing as much as possible. When you think it is enough for you to stop, then you should return to the primary object, the abdominal movement. Sometimes the sound may last for a second or two. Then, when the sound has disappeared, your mind will naturally go back to the primary object, rising and falling which you should note as usual.