Ok so when my car is idling, and I switch the aircon on, I hear a loud knocking/tapping noise. It looks like the serpentine belt tensioner starts 'jumping' back and forwards, and I'm pretty sure this is where the noise is coming from. If I give it a little rev, the noise stops until the revs drop again. If I turn the aircon off, the noise stops.

What I'm not sure of is what is causing this. Should I be looking at the aircon pump or the belt tensioner itself?

 

Chris, from what you are describing, it is sounding like it is the aircon pump that is the source of your issue. If you can, get the front end of the car up in the air on jackstands and then start the engine. Turn on the A/C and see what is jumping around on the front side of the engine (your driver's side). I think you will find that the pulley on the aircon pump is going to be jumping around too and that will be a sign that the aircon pump is toast.

The only other thing that I can think of is one of the idler pulleys is starting to go and it is sticking, but only under heavy load. You can try starting the engine and then turning on a lot of electrical loads (seat heaters, rear defroster, headlights, and dash blower (not on A/C or defrost as both of these activate the A/C system)). If you have knocking then, you most likely have an idler pulley issue. This is where watching the belt and seeing where it is vibrating should help you narrow down what pulley is sticking on you as the belt will be fairly steady upstream of the pulley, but very wobbly downstream.

 

Chris another thing to check will be the crank pulley itself, they are known to start coming apart on the diesel 2.0/2.2 engines.

The extra load put on the belt with the aircon switched on is what makes the noise.

There is an uprated pulley kit available from Jaguar that comes with a new belt and tensioner etc.