I was expecting some comments about changing the gasket when there was no need to! During disassembly, the hole in the rubber hose on secondary air injection should've been a giveaway - if only I knew what it was for at the time!
Thinking about it, the smoke on initial startup must be unrelated, but the noise appearing after 20 mins must be the amount of time it's taken for the hot gases to finally breach the rubber and make the hole. It has also melted the rubber under the quick connectors connecting to metal pipe, compromising integrity of connection, nevermind the hole!
Right, it has the standard exhaust, except a big twin 'D' shaped stainless steel tipped back box, embossed with Irmscher. I'm assuming this is factory fit, or aftermarket OEM, I didn't ask when I bought the car, but the box doesn't fit exactly 100% right in the cutout in rear bumper. With car running, but not hot, I can feel gases being forced out the secondary air injection pipe, from o/s bank back towards the front of car (this is wrong I take it), although I am able to cover the hole with my finger to stop gases without pressure being to great on my finger.
Is the rubber pipe on secondary air injection a high heat hose, or I take it it must just be able to stand radiated heat from the engine bay? Motorfactors just gave me a scared kid look and didn't know what i was talking about! I've got flexible copper pipe from B and Q that I was going to use to bridge the gap, but from what MDTM says, it sounds like i must need to investigate possibility of exhaust blockage before going down this route. Which comes onto my next question! Is there an easier way, apart from disconnecting an exhaust section at a time to measure pressure to try and locate the blockage? I don't think so but I hope so!
Incidentally, brakes seem worse again with pipe with hole in - might be something to do with vacuum?!? I don't know.
On the plus side i took some photos when doing this so will attempt at making some sort of guide.
Line up if you want to call me a */'"-$!^ for changing exhaust manifold when I didn't need to! I think 20 nm for nuts is what it said in Haynes, although that goes to T mines a W, but can't be much difference?