Car is making strange noise

Hello everyone.

I have a Diesel Renault Megane (2001). Recently it started making a strange noise (kinda like a fan, but higher pitch, like the wind blowing through a small space). This noise only occurs when i am accelerating with a gear on, and usually is louder when rotations start to get higher. Also, it gets quieter when the car gets hotter, but it is still audible. I dont know much about cars and i am very worried because despite being an old car, i like driving it, and i cant afford a new one.