My GLC also developed a fairly constant creaking noise from the suspension about 3 months ago. Not just at speed bumps, but in normal driving.
It takes at least a couple of weeks to get a service appointment at my local MB dealer, so it went in a few weeks ago to get this sorted, as well as one of the door handle LEDs which had gone.
They returned it at the end of the say saying yeah, there's a TIPS document about this, it's a known issue. The fix is to order some grease, plus 4 or 5 hours labour to "fit" it. And we have to order the LED for the door handle which takes at least a week.
Wasn't best pleased, as they had 2 weeks' notice of these problems in the first place and did nothing about ordering parts. They said yes but they had to see the problem themselves before they could order parts.
So, going back in today to get the grease applied (and LED replaced, hopefully). Will let you know if that cures the constant creaking.