Your system has multiple fans; it may have one case fan, but there are also GPU fans that can make this same type of noise. You gotta narrow it down, but it shouldn't take too much time.
I agree with Tweakbl
's analysis -- it really does sound like a fan and not a hard disk. I just replace fans rather than oil them, because more often than not the manufacturer doesn't provide a hole for access to the sleeve bearing (no reason for this either, other than to make you buy replacement parts rather than service/repair existing ones).--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.