help with techincal assistance

[mpj29201]

Hi all
Can you help me with a problem, please. 
The part (ringed in black) in the attached image is making a sort of whistling / buzzing noise when I turn the Ignition on? 
If it has anything to do with it, I had the cam belt / water pump changed a couple of weeks ago

Can you tell me what it is, and why might it be making this noise?

It would certainly help if I knew what it did, Help please!

Thanks
Life is so tiresome