Dog barks non-stop in the rally ring

I started competing in rally with my dog when he had just turned 1 and he did extremely well. Got 1st and 2nd places and almost perfect scores (99/100) in his first event and got his RN title already within one weekends show. Always walked in a perfect heel with constant focus on me and never had any issues with getting distracted.

He's 1.5 now and we're having some trouble in Advanced as he's recently started incessantly barking in the ring. He never used to do this but now will bark non-stop from the start sign to the finish sign. Initially he only did this in the ring at shows but now he's started to do it at training as well.

I'm lucky and the judges have been pretty lenient and haven't deducted many points for it, I assume in part because he is a small spitz and his breed has the reputation of being very vocal (he's overall very quiet for a spitz though I should note). However, it is affecting his performance as even though he'll do all the signs still no problem, he doesn't have the same level of focus and precision so we keep losing points for wides and wonky sits as he's too busy barking to stay properly against my leg and now only looks at me when we get to each sign. This is a dog who has done quite well and placed in most competitive obedience shows he's done as well so the little bastard definitely knows how to do a proper heel. Unfortunately he's started occasionally barking in obedience too although nowhere near as incessant as in rally…

I'm not sure if it's from excitement or frustration but he won't shut up. I'd lean towards excitement though as he whines non-stop out of excitement in the car whenever he thinks we're going to training and whilst waiting for his turn. He definitely loves rally more than obedience so I guess that could be why the barking isn't such an issue for him in the latter.

How do I get rid of this behaviour? Telling him off has no effect and I've tried just stopping every time he barks but he'll shut up only to immediately start up again the second we start moving. Any advice would be much appreciated!