When I fold or push my stomach I hear lyk a crackling or drum sound...sometimes there's air or gas inside. Why is that?

Normal. There is some air/gas in the stomach and intestine in all people and it is normal. Some of it is swallowed air, some is gas produced by the bacteria in the intestine. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Drink enough water daily so that your urine is mostly colorless. Practice safe sex. Get HPV vaccine.