I have owned my fiesta st for 2 weeks now, first week was fine really pleased, but this second week the car seems to be getting a lot noiser i.e. more boomier under acceleration.
Plus it also seems to have lost performance when accelerating hard.
Anyone have any advice, or am i just paranoid?

THANKS

You might just be becoming used to the car,I felt the same at first but after a week it felt slower though I was going faster.

NB: I'm not telling you how to drive the ST,, just explaining my experiences.

I too had the same experience,, i had to put my foot down once to avoid an oncoming car and there was an enormous lack of power for a car that produces 150 bhp. So I took it out for a little "head to head" to find out where the power band was etc,,,,,
The fly-by-wire throttle wont give you an instant throttle response, so i "Tease it" to find out where it bites. After 4000 revs (tis also here that the engine noise changes) the throttle felt more responsive and reacted quicker to a Kick Down.
It LOVES the petrol and the more you give it the happier it is. I use 75% throttle on acceleration ,,, leaving the remainder to finish off just before the gear change. Don't be scared to put your foot down during high revs,,, the ST will surprise you with that last little burst!!!

I found , the knack is finding where in the rev range, you're best putting your foot down according to your driving manor.

Don't want to " Teach you to suck eggs" but like i said,, i was disappointed with the lack of power at first, also.

The ST needs to be revved pretty hard to get it going, its not a turbo/diesel so you can't expect massive urge much below 4k rpm. You said you've only had 2 weeks too, the ST does also need a while before the engine properly loosens up, once you get past about 6k miles you'll find it feels a lot more responsive (mine certainly does).