Barking in crate - any ideas

[ijustwant8hours]

Six and a half month old border terrier has now been barking in his crate for half an hour non stop, he has been out so it isn't that. He has been crate trained since 8 weeks without any problems. He has been barking more generally but not usually too much in his crate (he usually just goes to sleep), and not for this long non-stop.

I am in a terraced house so I can't let it go on much longer...

Any ideas on why he might be doing it? After getting to six months I really don't want to have to sleep by the crate now!

When SpicyPup occasionally barks in the crate it's because he's heard something to set him off, like church bells, fox or the police helicopter. Anything like that possible?

Why is the dog in the crate?

Thanks Spicy, I haven't heard anything unusual - but maybe he has? He seems to have quietened down now...I am just a bit worried that the barking will be self reinforcing or something (he hasn't been crying or scratching, just barking)