Recently I've noticed my rabbit has nose breathing whistling noises when he is sleeping. I paid attention to him when he was awake but didn't hear anything only he was making these sounds while sleeping and it was very low noises like I only heard the noises when I put my ear near his nose. He just fine eating, drinking and playing as usual and there is no discharge in his nose or eyes and he coughs once in a while. He is almost six years old. Should I be concerned about the faint noises or he is only snoring?