Dr_b_ said:
What is the make/model of the PSU and how many Watts? What are your system specs? if it isn't fan noise or something hitting fan blades which is what it sorta sounds like, and it is the PSU, it doesn't sound healthy.
I was thinking that myself, sounds like a cable hitting the fan blades. I have had a PSU though that buzzed a bit, and It didn't last very long after it. Make sure you are not running the PSU at it's fullest, and when you buy a new one try to give it an extra 200 - 300w. This will help it to last longer and give you some future proofing.
Edit, I never noticed it was a 750W, damn that should be ample, maybe it's just seen it's days.