Moaning?

My little boy is 5 months and he literally makes a moaning noise all day unless feeding or asleep. I mentioned it to the doctor and he doesn't seem concerned as he said baby is in good health so what could it be!? I don't know if that's him trying to babble or if hes bored all day, teething or frustrated??, I'm at a bit of a loss really as he did used to be very happy. Its sounds like "mmmmmnnnggg" anyone elses baby do this? He doesn't cry though? Just makes that noise...