The coolant reservoir on mine does not vibrate at all. I hit it pretty hard and it is study. I was looking at the piece of metal under the cat. It is only held in by 2 bolts and does have a little vibration when you whack it. Also the wheel well inner location has some play too. Just some thoughts. I have the clunk too, but mine happens even whe not going over speed bumps at slow speed. <15mph.
White Platinum Tricoat Metallic
2012 Ford Focus Titanium
Handling Package Nitto Motivo's
Borla Exhaust with UltraQuiet resonator
Injen SRI Intake. Steeda RSB