Loud knocking when cold starting. (Vid inside)
 

Video of noise

I'm trying to diagnose where a noise is emitting from when I cold start my car. The noise goes within seconds of the engine starting and isn't apparent when fully warmed up or during hot re-starts.

The sound can only be described as a rod or piston wanting to jump out of the engine, but only on cold start ups? From the driving seat it sounds like it's coming from the intake side of the engine. The car drives perfectly fine, and boosts well.
It's almost like the engine wants to throw something off that is loosely bolted to it?

Things I've quickly checked:
The exhaust seems to have no excessive play in it but I've not had it in the air yet.
I did a compression test and results were fine cylinders 1-4 resulted with 192, 196, 189, 191 psi.
All spark plugs are a nice tan colour.
I've checked the crank pulley for wobble.
I've also checked the throttle body butterfly on Richy's advice for impending implosion and it's rock solid with zero play.
I've checked the intake manifold and brace bar.

Things to check:
Starter motor?
Alternator being secure to block?

During the compression test, the starter sounded really rough and weak when rotating the engine.

Basically tell me that it's NOT big ends or some other major engine internal and I'll love you for a bit. :D

Motor mounts?

Wow, weird.

If it was a big end, it would probably knock for longer than a few seconds.

The fact that it goes away quickly tends to indicate that whatever is causing it is as a result of a lack of lubrication. Once the oil pump gets oil to the location, the noise stops.

How many miles has the engine done? Has it ever been apart? Could it be piston slap or something?

Yeah I've been told on the UK Nutz site that it sounds like the exhaust knocking. It sounds like this to me too, but seems to come from the engine bay. The fact it doesn't knock when re-started hot suggests something is expanding and stopping the noise occurance?

The mileage is 58k miles, and motor (internally) is stock.

I'm going to check the mounts and lift the car and check the exhaust properly. I don't like the way this noise has suddenly appeared out of the blue.

Freakin heck, just watched the video... that isn't piston slap. Sheez!

So you have an early NB... have you had the #4 thrust bearing checked? Probably would have happened early in the car's life so this probably isn't the problem but you should check it anyway.

You might have a valve sticking open, but ours is a non-interference engine so this probably wouldn't be causing the noise on its own.

Whatever it is sounds BAD. When you figure it out i'd love to know what it is.

Have a look at your exhaust, it could be knocking to the chassis somewhere - I had something like this once. Sounded like that!
It just knocks at startup because the engine is moving quite a bit at this ...

Well I've found the source of the problem. It's nothing serious Phew!

One of the intercooler mounts had sheared on the outlet side. This caused the intercooler and intake tract to become a little loose around the throttle body inlet and allowed my blow off valve silicone tee to wobble enough under cranking which caused it to hit the underside of the bonnet.

Broken intercooler bolt.
Attachment 194004
It's pulled itself out of the body, probably through stress (and I didn't rivet the TDR plates to the stock welded nuts in order to strengthen them :doh: )
Attachment 194005
Attachment 194006

I've temporarily fitted a suspended steel cable to hold up the intercooler on the outlet side until I can source some suitable fixtures to perma-mount it.
Attachment 194007

I also rotated the BOV and tee from here:
Attachment 194008

To Here:
Attachment 194009

No more noises!
The cold start vs. hot start was a red herring. Thinking back, I've realised I only ever cold started the car with the bonnet shut. When trying hot starts I was always in my garage with the bonnet propped open trying to find the source of the knock.

Thanks for all the suggestions guys.