It sounds like a minor obstruction near one of the fans.

If you haven't blown out the dust in awhile, you might consider getting a can of air, or if your vacuum reverses, blow it out.

• Be sure the power is turned off, and the power cord is removed.

• Ground yourself by touching something grounded, or wear a ground strap.

• Open up the side of the cpu housing that is opposite side of where the motherboard sits.

• Blow out the dust, then reverse the steps to return the PC to operation.

If you hear a fan blade click or see it being obstructed while blowing, there's where your problem is. Make a note of which component where the fan is attached, and see if it happens again. You might need to have that part replaced/repaired.

I hope this helps.