Area-51 R2, power supply

Hi! I have a 5 years old alienware area 51, I've already changed some components but this time it seems that the power supply has some problems since it started to make loud noises when the gpu is working hard (when I play games mainly, not for regular work). So at the beginning I thought it was the gpu but then, opening the PC, I've noticed that the sound comes from the power supply and I supposed that it's simply because the gpu is the most consuming component and so when it works it also put under pressure the power supply. My question are:

The sound is similar to a coil whine and it changes very often, it has never done so before; should I worry about the integrity of the power supply? (Since I've noticed it I simply stopped playing games or anything which could stress the GPU.)

Looking online I couldn't find any available replacement for it, where could I find one? And if I don't could I use another power supply with similar stats? (how can I check compatibility?)

Thank you!!