You could poke around with a stick with one end to your ear. Maybe you can pinpoint better the source of the noise.

François

 

RegT - did you ever find out what caused this noise in your D2?

 

Great great video.
You caught the noise
That noise is always around when you FIRST start the truck in the morning.
With the engine DEAD cold.
BOTH my Discovery I and my Discovery II make that exact noise.
Goes away in about 3 minutes.

Listening to it - may be pre detonation or pinging in the cylinders.
Because - the noise ONLY happens upon acceleration or on the increasing end of the RPM when you are going from low to high RPM or revving the engine from low to high.
This is the ONLY time the sound happens.

For this sound to even happen at all:

1) First thing in the morning.
2) Engine revs from ldle to 1,500 RPM or so.

Once the engine gets to 1,500 RPM - noise gone.

My Discovery I has this noise very morning for the past 12 years.
It is still going at 165,000.

My Discovery II has it at almost 100,000 miles.

Discovery I has GEMS
Discovery II has Bosch.

So to have both of those pre detonate at startup is unlikely.
Plus to pre-detonate you need a lot of heat already in a hot engine.

Still - if the timing is advanced ..
Would be interesting to put a mark on the harmonic balancer in chalk or paint
and see if the timing changes greatly when this noise is present to when the engine heats up.

Sort of a rapping noise. Has the sound of a clattering piston.
IF the engine is really cold - will the piston clank in the bore just a bit to make this noise?

If this is true, the noise will be extended in the winter - around 20 degrees or so.
Does not get that cold ever where I have owned my Land Rover.

 

This may be also called piston slap.
Other vehicles have it too

The infamous "piston slap"

Summary

• Description: Clicking, tapping, or knocking noise eminating from the engine after a cold start. The noise lasts until the engine is warmed, usually 5-15 minutes. The sound may be heard in the passenger compartment.
• Affects: 2.5L DOHC engine, particularly from 1997 to 1999.
• Diagnosis: The pistons are engineered for operation at temperature. In an effort to save weight in the engine, the piston skirts were shaved down. When the pistons are cool, however, they do not have sufficient contact with the cylinder to keep from 'slapping' or rattling against the cylinder sides. At temperature, due to thermal expansion, the pistons match up properly with the cylinder and do not rattle.
• Reported solutions: In some cases, SOA has rebuilt the engine short block, while in others SOA prescribed "countermeasure pistons" with longer skirts that do not slap. Most dealers will initially state the sound is "normal."
• Detrimental effects: SOA has said that the noise causes no engine problems or abnormal engine wear; Subaru engine expert CCR Engines of Colorado has said that the engine may experience a 10% reduction in life.

 

That was, in fact, one of my guesses - piston slap, when the cold piston is knocking in the bore because it has not yet fully expanded with heat to close the gap between it and the steel liner (which expands with heat at a different rate).

Mostly, however, I'd just love the original poster to follow up on this to let us know what the cause of the noise ended up being. I can't tell you how many threads I've read where I want to know the outcome. I see reply after reply after reply, and I think..."great, the answer will be in the next reply!" And then the discussion ends. It's like hearing a great joke, and then the joke teller leaves before they get to the punchline!

PLEASE - if you ask a question and your problem got resolved, come back and let everyone know what the cause and solution ended up being!

 

This video claims to diagnose piston wrist pin knock..

This knock sounds a bit like the Cold start knock we get..