Had this rattle from aux side for about a month or so now, have looked and checked the bottom pulley and all seemed ok, so put it down to normal TDCI noise.

Well Monday, started my trip home from work, changed to second and the rattle, tappy sound was not a nice sound, wasnt loud, but sounded like something hitting. stopped the car and was ticking over fine, had a very faint tinny rattle.
So drove home steady, but it had lost a bit of torque. The turbo still boosted as it did.

I have a feeling the chain tensioner has or is failing, and it lost tension when i changed gear and jumped a tooth.
My old focus did the same, the tension wheel was going, it felt gutless, had that in the garage for 3 days, to be told nothing wrong after changing plugs etc etc. in the end that let go and bang went 8 valves.

So if it was out by 1 tooth, would it still drive? still tick over smooth? or could it be something else?

I have had enough with it now TBH, it may be stripped or scrapped.