Car has just started making a funny noise, any ideas?

From London, 1 post

My Fiesta has just started to make a strange noise when I drive it, the noise is there all the time apart from when I slow down and it sounds like the car is struggling / almost revving. The noise started about five days ago and hasn't stopped.

I have checked the oil, water etc.

Anyone have any ideas? Should I be worried?

Vic