I sometimes get this here in the UK. We always dry our clothes inside as we don't have enough outdoor space for a line, but I find them get a bit of a smell to them either when they takes ages to dry - i.e. a couple of days when it's freezing cold like it is at the moment, or when I leave them in the machine too long before taking them out to dry them.
It sounds like there could be a whole host of reasons though so maybe go through the different possibilities one at a time until you find something that works.