Have you tried the listening stick test to isolate it?
You can use a long screwdriver.

 

Tried everything , used a mechanics stethoscope to no avail

 

I can hear a slight tick but it sounds like injector tick. Are you sure it has developed suddenly... rather than just one day you noticed it and then it forever annoyed you and now you are going looking for problems?

Aside from that, it sounds healthy enough. There are no tappets, you have hydraulic lifters.

Also, you are missing the undertray / wheel guards, those also cut down the amount of noise that comes from the engine!

 

Ok. It's a bit difficult to identify from the video but my first instinct would lean towards one of the hydraulic valve lifters failing.
The chain that connects the cams together should make a more contiguous rattle sound if it was worn rather than a tick.
It's virtually impossible to identify an individual valve lifter without running the engine with the rocker cover off.
Use the listening stick (or stethoscope as you have done) to identify it but it's messy as the oil goes everywhere.
Edit. BiGK posted just ahead of me so go with his suggestion first,
I'd still recommend using a big screwdriver as a listening stick. You can locate noises to within 10mm with that.

 

the tick started as a now and again thing , gradually got worse over a few months , im going to try some new injector seals this weekend , if no improvement im going to try some liquid moly hydraulic treatment see if it quietens down , the noise only appears when its hot which is strange , other things im thinking is crank pulley or maybe a dodgy water pump ? its a definite loud tick at idle . so far this car has has mobil 1 esp 5w30 oil , filter , new injectors , egr strippped and cleaned and fuel filter

 

Injector seals won't help, you should be identify failing seals from either a pool of diesel forming at the injector base... or the black death.

Did your car have the tick BEFORE changing the injectors? Why did you fit new injectors?

Also why are you changing the engine oil every 3000 miles?

 

I had the tick way after fitting new injectors , started as a sometime tick to always ticks when engine is above 70 degrees , I change the oil every 3 to 4 thousand miles due to lots of short journeys so don't want it thinning out during regen , noticed last few days the tick is at startup too so either thinking crank pulley which looks fine and is a decoupled one or bad hydraulic lifters ?

 

You would only ever thin out the engine oil if you had repeated failed regen's. Also, I don't know exactly the injection strategy PSA uses but as the 407 does use eolys fluid to lower the combustion temperature, the engine might not use a post-combustion injection of diesel to essentially light a fire in the DPF. i've never heard of anyone suffering from oil thinning. If you are that concerned, you can always pay around £30 for an oil analysis.

What fuel do you fill up with? On mine, I notice that supermarket diesel makes my injectors tick a fair bit more!

 

i use texaco fuel , tried using premium diesel on a tank full and made no change, as far as i'm aware the 407 doses the tank with eloys every time you open and close the fuel door, and during regen injects at a longer duration and post injects to heat up the catalyst . i've just added a tin of liquid moly hydraulic lifter additive to my oil , going to see if anything changes in the next week or so . this ticking/tapping sounds like when the high pressure pump cross is worn but its brand new , less than a year old . i'm only concerned that this tick will turn to a knock and shaft my engine.