2009 E350 Sport Coupe - Knock at the Rear going over Speed Bumps !!!!

Hiya All

Please Help.

I have just acquired My 2009 E Class Sport Coupe and although at normal speeds the Suspension seems fine, at Low Speeds when going over a speed Bump there seems like a slight knock coming from the rear tyres, can't make out if there's a fault or the Tyres are just hard

Please Help