Ignition off sequence 1.6 diesel

Hi there recently acquired a 2011 1.6 diesel. Not sure if this is normal but after turning off and removing key a buzzing/ humming  continues for approx 20 seconds ,stops and then dash finally goes off. To be a bit more specific buzzing noise is a bit like an injection pump or similar type noise. Bought a service manual to try understand shutdown sequence better but has insufficient information. Any advice appreciated thank you