Component is making buzzing noise
Figure 1. Side-view of BBC B1028.



Figure 2. Top-view of BBC B1028. This seems to be a panel-mount variation. The OP photo shows a DIN mount.

This is a relay or contactor for 1 or 3-phase switching.


  The one of the right has been making a "buzzing" noise for the past couple of weeks. Why is that?


The relay uses an electro-magnetic solenoid to move the contacts against the opposing spring force. When the moving armature contacts the coil core there can often be a bit of mechanical vibration. Sometimes a thump can fix it but it will probably return at next switch on.