Anyone else buy a NZXT 140mm fan which now is rattling?

Some fans are designed to be mounted certain ways. Different type of sleeve on the fan means it works better at different angles. search the threads on a guide and you will come up with a few good answers

you might need a different brand, where did you put the fan?

Rifle bearings are a type of sleeve bearing that while much improved can still have some of the same issues.
Try a few drops of light machine oil into the bearing housing, which you gently peel off the sticker to get to (keep the sticker, it must go back on). Then after its run for a few days, you can add a bit of light grease which will be longer lasting.

I have found that the absolute best fluids for fans of all types (sleeve/rifle/ball+sleeve/2ball) is synthetic motor oil, specifically a Group IV or V oil, and more specifically Royal Purple or Redline. I run both oils in my track and street cars (the stuff is truly the absolute best motor oil, redline is, with RP a close second place), so I figured if it can work in a 180hp-per-liter 10.5:1CR BMW I6 engine with a Lysholm twin-Screw Supercharger pushing up to 21psi, it can work in an itty bitty fan

I just grabbed some from the garage, specifically 0W40, and I have 8yr old Thermaltake and Masscool sleeve-bearing fans that are dead-silent to this day, and I actually noticed that after turning off the computer, they spin about twice as long before stopping.
I have used, specifically: Redline 0W30/0W40/ATF4/75W90(gear oil) and Royal Purple 5W30/0W30/5W40 and found that the 0W40 seems to work the best. I have an infrared thermometer and the fan motor hubs even dropped 2-5C, with ball bearing fans showing the biggest drop in temps and biggest noise reduction.

Unlike machine oil, I have done this twice over five years and after the first 3 when I changed it the oil came out looking brand new, no black buildup.

Plus, you can use the rest for your car, the stuff will clean your engine like you won't believe (I did a full teardown of my BMWs engine about 20K miles after having it fully-built and the SC installed, and the engine was not only spotless, there wasn't a single sign of wear anywhere, normally the cams get marked up as do the piston rod arm bearings but nothing; my mechanic/friend actually checked the odometer to make sure it was functioning, and he's the same one who helped with the initial install) - point being, it's seriously good stuff.

If you have any sleeve bearing fans left, lubricate them as nleksan says -- but do it before you put them to work, and re-lube them periodically.

3-in-1 oil contains a penetrant. Not good for fans. Top up your fan every few months with some real oil. Motor oil will do. See my sig.