Increased respiratory effort, rate, and/or noisy breathing, can be indications of a partial obstruction, possible respiratory infection including pneumonia, or even possibly a heart condition or a mass in the chest cavity. I would strongly suggest contacting your vet right away. If your vet doesn't have an out of hours emergency number and you don't have access to an emergency vet that treats rabbits, I would in the very least make sure to call first thing when the vet office opens, tell them your rabbit is having breathing difficulties, and request your rabbit be seen immediately as an emergency.
http://www.medirabbit.com/EN/Respiratory/Differential/respira_diff.htm (noisy breathing in rabbits)
Medirabbit (respiratory disease in rabbits)
MediRabbit (pneumonia in rabbits, contains medical related photos)
Medirabbit (congestive heart failure in rabbits, contains medical related photos)
https://rabbit.org/vet-listings/