I thought it could be the EGR as I've had it off recently to de-coke but it doesn't appear to be letting by, although it does kinda have that 'hole in the exhaust right near the head' sound. My first thoughts before I even popped the bonnet was that it was a heat shield, It's a metallicy-ish sound.
I've left the car at home (working today) as I'm reluctant to drive it. I noticed it's a little more sluggish than usual until the turbo kicks in, a little bit gutless? Only tried it the one time up to 2.5k rpm, didn't want to break it. Luckily dad has kindly loaned me his Mondo for the day. Not sure how much more 'told you not to get an Alfa' crap I can take though.
Anyway, from what I've read on here possible causes could be:
Big end has had it and it's piston slap - Worst case I guess.
Tappet - I'm using 0w40 mobil 1 oil at the minute and I rarely take it above 3k rpm. That oil is pretty thin already. How can I tell if it's this?
EGR valve - Don't think it's this but will need to look again in daylight when I'm not soaking wet and freezing.
Cam chain - Dad is convinced its this although I'm not convinced it's the noise of a chain slapping around. The noise does rise and fall with the revs though.
Fractured rocker - Joss, you say this. Is this a fault you've heard of before on this engine?
Dmf hitting the gearbox casing - Don't think it's this.
Is there anything else it could be, any more suggestions? Still can't 100% pin point where the sound is coming from. It's either the top right hand side of the engine or middle left at the back, intake side ....depends where you stand over the engine to listen. Whatever it is it's not leaping out as an obvious fault to me but I really need daylight to have a look. What I can't get my head around is that at idle around 800rpm you can't hear it but throttle to just 900-1000rpm and it's really loud. My dad recommends using engine flush and putting new oil in it but I think engine flush could do more harm than good (138k miles) and I'm reluctant to sling £45 of oil in it without a bit more investigating (will have to be boxing day now).
How big a job is it just to take the top cam cover off to have a look at the chain/rockers? Any special care needed when disconnecting the fuel rail (is it likely to have any residual pressure. How do you bleed it etc?). Is there a numpty guide anywhere?