is it normal for a desktop computer to make some noise

i recently had to take my computer in for repairs, the power supply unit was changed as it went faulty, and my hard drive was changed to a new hard drive as the one i had went faulty....so i have my computer back now but, its been on hours now and its making a buzzing noise? or humming noise...it can't be the hard drive or the power supply unit...so what can it be? and is it normal?