Since the only moving part inside the PSU is the fan that’s really the only likely thing to be making a rattling noise. A cheap sleeve bearing fan can fail at any time…I just returned a brand new PSU with one that was already starting to rattle after only 40 hours or so heavy use. Needless to say, I picked a unit with a hydro-bearing fan to replace it.
You don’t need to worry until the fan stops altogether…or it gets so loud it drives you batty. Since yours isn’t brand new, replacing the fan is an option that’s actually a good choice because you can upgrade the fan to a ball-bearing or hydro bearing while keeping the now burned-in rest of it. But only if you’re techie enough to DIY the job, otherwise taking it to a computer shop will be more expensive than a brand new PSU. You’re next choice will be RMA’g it if in warranty…and a year later another rattling fan.