Nissan Almera poor fuel consuption and noisy engine when accelerating

Car just out of garage two weeks ago after having front breaks replaced and since then have noticed that my normal weekly £25 petrol is only lasting about half the week. The engine is noisy (sounds a bit like a 50cc motorbike) when accelerating past the big 2 on the rev thingy (as you can see i know nothing of cars and having been ripped off by kwik fit for my breaks - decided to check on here been googling it and getting loads of ideas) anybody every had this or know what is causing it, dont want to get ripped off again!

Thanks in Advance