My dwarf hamster whitey has always done this in his sleep too! He's over 2 now. I dont think its anything to worry about either. Just them sleep talking or something! I think its very cute
Edit: just thinking away with whitey he kind of has always done sort of soft squeeks that last a while when he is asleep. If its a sudden thing with your hamster and if the noise sounds almost like its coming from the chest , maybe get it checked up on at vet incase its a chest infection. My other hamster had a chest infection and would make chesty squeeks when sleeping and awake. Hope this helps