Replaced inverter to solve backlight, now it makes annoying noise.

I have a 2008 Black MacBook and the backlight started to fail randomly, I found out that I had to replace the inverter. I took My laptop to a repair shop and they replaced the inverter, the backlight in the screen works perfectly now.

The problem is; whenever the backlight is not at minimum or maximum, the inverter makes a noise, like a whistle, and it comes from the "axis" where the screen holds, so I assumed it is the new inverter that makes these noises. I told the guys at the shop and they have tried different inverters, which still did the same noise. They told me replacing the screen will solve the problem, but that costs 100€ and I wont invest this money on such an old laptop.

The noise responds to the level of light in the screen: the pitch gets higher and lower depending on the backlight.Also, for some positions of the screen the noise gets louder.

My question is: why does this happen? Is it that the inverters they are trying are cheap ones? Is it maybe that I have to replace the inverter cable too?

Thank you, and excuse my english.