2009 Corsa D 1.4 rattling engine part

My girlfriend’s Corsa is making a slightly strange ticking/rattling noise. I’ve found the cause to be these plastic parts (actuators for something?) and the metal clips that are retaining them. (behind the injector plugs; round metal clips)

You can’t see the parts too well in the zoomed out pic but they seem to be in line with the four cylinders.

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When I put pressure on the parts with my finger while the engine is idling, the noise greatly reduces.

Is this normal as the engines get older? Is there any way of reducing the rattle?
As an aside, what are these parts called/for?