Banging lorry side

The police were suspicious of a lorry coming from the docks so decided to follow. Every few miles the driver stopped got out and banged he lorry sides

This happened about three times before the police decided to pull him over.

Police to driver " Sir we have been following you and every so often you stop and bang the side of he lorry, why?"

Driver to police " Well officer it is like this, This lorry is a 7.5 tonne lorry and I have 10 tonnes of parrots inside. So by banging the lorry sides keeps them flying around and lightens the load"