[BiGK]

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!

 

[BiGK]

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?

 

[BiGK]

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!