IPhone constantly 'dings' when in docking station
iPhone intermittently goes 'ding' when in the docking station charger
The reason the phone make the ding notification sound is because it thinks that it has just been connected to an external power connector. If this happens intermittently, especially as you walk about it is because the connection is making and breaking as the phone is moved by vibrations.
This common problem is usually caused by a poor connection between the docking station and the iPhone, a common culprit is the male connector that extruded from the dock. This can usually be proved if you find the phone does not exhibit this problem if it is connected directly to the power connector.
Cleaning the male connector can improve this issue. If you have some liquid metal polish, unplug the charger, dip the tip of a cotton bud in the polish and use this to clean the connector in the base station. Polish with a clean cotton bud and ensure that no trace of the polish remains. Reconnect the charger and test it out.
I would never recommend attempting to clean the female connector within the iPhone, that is likely to cause expensive damage to your iPhone.