The original 12cm exhaust fan in my Antec case started to exhibit occasional rattling noises, prompting me to seek a solution by acquiring a new fan to replace it. I bought a Noctua fan (I already had an 8cm fan by Noctua). Damn, it's silent. I never realised how much that Antec fan had contributed to my computer's noise level, even when it was still new.

Quite a pricy fan, though. Actually very pricy considering it's just a fan. But the drop in the noise level removed any regrets I might have had.