A strange knocking sound at 2000 rpm, similar to a helicopter sound, can be heard inside the car, but barely outside. They appear when driving and at idle speeds in the region of 1600-2300 rpm. What could be the reason?

 

 

Bit more info would be helpull. What year your XJ is? What model of engine you have? Diesel? Petrol? If petrol what model? (i4, V6, V8, V8 SC)
The sound from drivers seat are very much toned down by sound insulation, so extremely hard to spot anything from that, but there are many possible causes starting from exhaust manifold leak, belt tensioner, EGR leak, up to the bearing or wrist pin failures.
Best way is to take the vehicle to any garage for quick checkout. (at least almost any garage can check if the noise comes from external or internal of the engine)

 

Car info as Vasara said above!
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Hook the car up to a working SDD and report back.
It could be anything or nothing?
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there are no errors in sdd

 

Well that's a good sign? I think?
Maybe something is loose and banging around under the car? Might be a simple fix. Can you get the car on a lift and start poking around underneath?
I listen to the video but really don't hear anything out of the ordinary?

If it was my car I would probably change the engine belts as first step because with the age of your car they are probably due anyway. With the sound related to RPM you might suspect a tensioner or idler pulley too?

It's better to not start multiple threads on the same problem as it creates confusion and clutter?
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