Not to beat a dead horse, but... BARKING!

[SolaMio]

I know this is probably the most common behavioural 'issue' people have with their dogs, but here goes.

Sola has started to drive us a bit foolish with the barking. I used to think it was pent up energy, but she's also barking even after a long (5 km) walk. She doesn't need food, water or to go out (though she barks the same way when she DOES need to go out, so it's hard to distinguish, except to say 'outside?' to which she goes to the door). She gets lots of attention and pats. I don't like that she is barking AT DH and I. We just can't figure out the WHY. It seems like she wants something but we don't know what the heck it is I'm sure she finds it enjoyable, but we alas do not

I do NOT want to do the shock collar (please don't pounce on me for bringing it up, I am pointing out that I do NOT want to use it even though numerous people have suggested it to me). However, someone else mentioned the citronella collar. I know it doesn't hurt the dog per se- but it's still negative reinforcement. We do use a lot of positive- but kind of hard to praise the dog every time she's not barking. Though, we do praise and reward her for being 'good' (calm and nice in the house).

I'm not sure what else I can give as background- she could be bored I suppose, but has toys and chews to play with... And we don't let her 'get away with' bad behaviours (jumping up for example, which I am SO trying to cure, but she is excitable and persistent). She also doesn't have full reign of the house (just one area at a time).

Any ideas? Sola has a pretty loud bark which I'm worried might start to bug our neighbors if they can hear it... And it's driving us foolish