Lenovo b40-30 All in one desktop weird sound when going to sleep mode

Whenever i go Fn + F3 (which is a shortcut for Sleep mode) or Win + X+U+S, the desktop will make a weird sound almost like the fan inside is overreving(like in a car) right before the PC goes to its initial sleeping state. What could this mean?

Ruing Windows 10, 64-bit Almost brand new used only for some time.