e6500 - High Frequency Noise

My e6500 worked silently over the last years (especially when HD was replaced by a SSD). A short time ago, after both ALT and ALT-GR keys failed on the original (blacklit) keyboard, I replaced it with a new one. since then, the laptop emits annoying high frequency noise, especially when idle.

when the old keyboard is installed, it still emits high frequency noise, but not as loud as with the new keyboard.

is there any change to get rid of the noise?