Yes, it's a fan bearing going, either on the power supply or the case fan or the CPU cooler. I've had this on an old PC when first switching on, the process is (a) screech from bearing (b) bearing generates heat (c) any residual lubricant is liquified by the heat and runs into bearing so screech stops. Very easy to deal with, either get a scrap fan from a PC repair shop or buy a new one (they are extremely cheap), and very easy to change.