When we shut the fridge door a loud continuous humming noise comes from the fridge. This seems to be activated by the door light switch at the top. Thank you

There are 2 possibilities I can think of for the hum. When you activate that door switch it activates 2 fans and the compressor. There is one fan located on the compressor and one located in the vent system in the fridge. The second fan is used to circulate cool air evenly through the fridge. I would recommend holding your finger on the door switch and listen carefully whether the noise is coming from inside the back of the fridge or from the lower bottom. Lower bottom could indicate compressor cooling fan or compressor. They would need to be inspected. If it sounds more like the other fan mentioned above it would again need to be inspected. The compressor cooling fans can get stuck or become noisy due to bearing failure. The vent fan can get stuck or noisy but also in some cases if the defrost circuit is not working can chock up with ice and create a buzzing noise as the fan blades strike the ice.