Well, all the time is an exaggeration; he doesn't tend to sing all the time at home unless there's not much else to do eg sitting in bed waiting for us to get him up!
But he started pre-school in September (turned 3 in December) and they've mentioned he sings/hums a lot. He was doing it a lot more at home before Christmas but it seems to have died down a lot. The songs/tunes he hums vary (it might be the same one for a few days, he sang the ABC song all last weekend when we were in the park and it drove me nuts!) but pre-school clearly think it's a concern.
He recently had a schedule of growth check with the health visitor who had no concerns.
Do any other children of a similar age do this? And more so in pre-school/nursery etc? At home it's really not a concern...