Replaced inverter, now makes noises.

MacBook 2008 OS 10.6.8

The backlight started to fail randomly so I took my MacBook to a repair shop and they replaced the inverter. The backlight works fine now.

The problem now is:

Whenever the backlight is not at minimum or maximum, the inverter makes a noise, like a whistle.

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 don't won't to 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.