It's not something I've experienced. That's the problem with intermittent issues they never seem to happen when it's at the dealership! Good idea trying to video it, at least the dealer can hear it happening..
Don't know what could "clonk" at the back, unless exhaust is wobbly, or a brake sticking and releasing?
GLC 350d AMG Line, Premium Plus, Polar White, Airmatic, Intelligent Lights, 20" Wheels, Towbar.