I noticed last night (about 11pm) that my bunny (an approx 3yr old Netherland Dwarf) was making a sort of whistling/squeaking noise when he breathed. The sounds is similar to the whistling when us humans get a blocked nose, or a wet cloth squeaking on glass, but very faint/quiet (I can only hear it when my ear is within 30cm of his nose).

There is no evidence of any discharge or gunk around his eyes or nose (he looks perfectly healthy) and he seems to be moving/eating/drinking etc normally. Rest assured if I see any signs of sneezing or discharge he will be going straight to the vet to get checked out.

This morning (8am) the whistling noise was still there when he was sniffing around waiting to be fed, and whilst eating. When he relaxed afterwards though I could no longer hear it. I’ve heard that bunnies can snore, but if he is doing it whilst eating this mustn’t be the case. I’m thinking perhaps he has always made this noise when breathing/sniffing heavily and I have just not noticed it before, is this possible?

His bedding is oaten hay (has been for about a year), and prior to that it was meadow hay. His diet has not changed recently aside from giving him a mineral lick (a brand he has not had before) 3-4 days ago.

Can someone please tell me if what I am describing is cause for concern, or if it is normal?
Is there anything I can/should do?
Also, I live in Australia and it is forecast to get to 38 degrees Celsius (about 100 Fahrenheit), would it be ok to still give him a frozen bottle (maybe on top of his cage) to keep him cool? Or is it likely to make it worse (especially if it is some kind of respiratory infection; I hope not)?
He is outdoors (we have indoor terrier-type dogs) but I move him into a cage in the garage on hot days so it is cooler (and he stays there overnight every night so that he is safe).