What Causes The Infant To Moan During Nights?

My 16 month old twin boy moans at night. Some nights only a slight moan then other nights it goes on for hours. He sleeps with us. The other twin sleeps on his own without issue. I would like to try anything that might work. Please give me advise. We have tried pain relief and it has not worked. Milk not worked . Massage and music have been tried. Thanks