Hosing the bike off as you did is fine, certainly not any worse than riding in the rain or through puddles would be, and mtn bikes certainly shouldn't have problems with that.
As to the noise, it's probably minor, and not a hub failure. Could just be some grit under a seal lip or the like. I wouldn't sweat it and given that the bike is new, I'd expect that the LBS would inspect and correct it under warranty. While there speak to them about what it turns out to be and what, if anything, you should do different in the future, and also about general care of your new bike.
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WARNING, I'm from New York. Thin skinned people should maintain safe distance.