Dogs can have what's called sinus arrhythmia. The heart beats faster when they inhale and slows when they exhale.
It's normal. Can't say that's what your dog has. A Vet could listen and decide, but often it's only present when the dog relaxed. I hear it a lot in sedated animals.
It never hurts to have a professional listen, so please don't take my word on your dogs cardiac health.
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