if you pop up your bonnet and you hear rattling what is that?

 

Could be a number of things where exactly is the noise coming from to narrow it down?

 

the bonnet catch is loose

You need to give a little more information if you realistically expect people to help.

 

Most likely cause is a heatshield, check the uppipe, downpipe, turbo and headers.

 

tappets,heatshield or piston deciding it wants a holiday like mine

 

As above, rattling most likely the heatshield, if it is a clicking then most likely the boost solenoid (assuming drivers side), ticking could also be noisy injectors and depending on the age of the car could also be hydraulic lifters, or clearances or piston slap........

Basically better description needed for a more accurate answer as there are loads possibilities