Just tried mine out. The spacebar is noisier than my other keys for sure, but not significantly enough to have made me take notice before. I've never actually noticed it until you just pointed it out right now. And come to think of it, every keyboard that I can remember using has had a noisier spacebar in comparison to the other keys...
Is yours noisy enough to warrant a replacement? If so, take it to an Apple store and compare it to other keyboards there and possibly talk to somebody to have it swapped out.