engine rattle

[lackofgrip]

when i press on the arrowed thing in the picture the rattle goes. how much is this to replace and whats the part called????

 

[Parrot of Doom]

Looks like a belt tensioner shocker, and it'll probably cost no more than £15.

Bear in mind that the engine mounts may be on their way out - engine vibration makes these things wear out more quickly. This was the case with mine.