Hey everyone!
I’m getting a strange noise during cold starts, it lasts for about a minute and then goes away.

Please check out the sound in mp3 file attached till the end, you can clearly hear it there.

Is this a normal sound, or should I be worried about something specific?

Thanks!

C250, 2011 4MATIC.

Have you checked the hyper drive? EGR valve? Hopefully someone who actually knows something will chime in.

Could be the secondary air pump.

Thanks a lot for your advice!
After doing some research and listening closely to where the sound comes from, I’m almost certain it’s the air pump. Technically, it’s working fine, no error codes or anything and it shuts off on its own after a minute. It probably just kicks in when the temperature outside is around freezing.

Thank again!

Mine didn't throw a CEL or a code, but it howled like a banshee. Fairly easy to replace. Good luck.

Oh interesting... Just out of curiosity, did it make any noise after you replaced it?

If it is the secondary air pump it does sound a bit off.

It should turn on at cold starts and run for 40 seconds. Its for emissions to get the cats hot quickly.

Depending what obd scanner you have you should be able to function test it. If you do the manual says dont run it longer than 2 minutes.

After replacement, the new pump was significantly quieter, than the original unit. I could hear the new pump, but likely because I was listening for it. If I didn’t know about it, it would be masked by the noise of the higher engine idle speed on start-up.