strange short grinding noise

I have a e200 kompressor 2002. It make a short (0.25sec) grinding noise as the rev counter pass 1250rpm, and then as the rpm's decrease at about the same rpm's.

It sound af if the sound comes from under the red cover plate over the spark plugs (with the marking Kompressor). It makes the noise every time I press and depress the gass peddle. Even when I pull the cable inside the engin compartment by hand and in Park or drive.

It sound more like something is engaging and disengaging when the sound is generated, and not as if it is a bearing on a shaft that grinds on a specific rpm.

help would be appreciated as this sound is annoying, it is not loud from inside the car, but with the bonnet open it is alarming.

I know this is not the W210 forum, but I thought that there will be more kompressors here.

Sorry for my English, not my first language.

Charl