I just realized my ASUS is way older than yours and yours has that dual fan setup I'd forgotten about. And it was your thought that it was some kind of whirling noise which had me focused on either fan or HDD.
Fan failure normally happens in a way that doesn't allow for off and on noise. They normally just flat out make lots of noise and then quit.
An HDD, on the other hand, can be very odd with noise. You can hear something and then it goes away for any number of reasons and that includes just jiggling the unit around, and even transporting it to that computer shop could jiggle some part back into a good setting long enough to pass a quick check by the shop owner/employee and think it is fine, but then later it starts acting odd again.
But you have DaveM121 offering good advice, so go with that.
But an HDD failure can't really be diagnosed simply by the fact that it seems to be working fine at this or that time and under this or that sort of load. Even sound doesn't always give you warning. But that the sound you are hearing is intermittent gives me a sense you can remove either of the fans as being the trouble, unless it is something sort of stuck in a vent, BUT the shop would have seen that and have removed that and the sound should not have returned. I'd go back to your original HDD idea and not trust that it seems to be okay.
But try out DaveM121's advice, too. I'm sure he is better at this troubleshooting stuff than I am.