The cause to this problem is the damper mechanism in the muffler. European Countries has strict sound regulations which requires that the vehicle engine running is also quiet. In the muffler there is a damper plate that is vacuum line powered. When the engine is running at low RPM conditions, the damper is closed which is controlled by the vacuum from the engine. As the engine power is increased, the damper opens under vacuum. As the vehicle ages, the vacuum line can leak due to drying out of the line material. The rattle that you are hearing may be due to the vacuum line leaking such that the damper cannot come fully open when it needs to so that it rattles on the stops.
With your vehicle off, you can look into the exhaust pipes and see that one of them has a metal plate blocking it closed. Start the vehicle and have another person pulse the throttle while you stand behind the vehicle and look into the pipe while the engine is running, you will see the damper plate open and close.