What could be this sound (attached)?

Dear All,

After purchase of my E200 (mopf, petrol, 24000km, after MB maintenance at 23500km) I recognized that its engine makes unexpected, IMHO, sound when it is cold. It sounds like metal knocking+a bit of yelping.

The sound disappears after warming up.

I don't remember that the sound appeared when the car was started up in the dealer's showroom at +17...+18C, but it is repeatable and recognizable around 0C.

What could this be? Its rate depends on the engine's rpm, but a few times less than the rpm.

You may find the sound attached in mp3 format. It was recorded inside my garage box this morning.

By the way, surprisingly I found that this higher frequency sound may not be heard on some PC players ...