Crackling noise under bonnet?

I'm sorry but the sound file doesn't appear to have uploaded. Could you please try again?

I don't really hear much over the diesel, but there is a sort of metallic ting I can hear. It almost sounds like a bearing starting to fail. What I would do is to let it cool down, now remove the accessory drive belt and go to each of the pulleys it runs on and spin them with your hand. Listen for anything that sounds like grinding and feel as you rotate them, they should all feel smooth and quiet. Now start the engine and listen for the sound with the belt off. Do you still hear it? If not check the belt driven pieces close. It could be a bad bearing in an idler or tensioner.
If you still hear it with the belt off shut it off and get a mechanic's stethoscope and then use that to track down where the noise is the loudest.

It would depend on what it actually is. A bad bearing could fail and cause the belt to fail and need to be towed. However anything would be a guess without actual diagnostic.

Yes, it could be the bearing in the clutch or the compressor itself. There are multiple bearings in them. If you don't have a stethoscope there is another way that can at least point you in a direction. Use a piece of hose (like garden hose or similar) hold one open end up tight to your ear and then use the other end to listen around the parts to see if you can home in on the likely problem. If it is the A/C and only happens with the A/C on you might try a test, with everything off reach in and disconnect the electrical connector that will be on the clutch assembly. Now start it and see if the sound is gone. If it is, you have your answer. Then it comes down to replacing the bad parts.