Ford ka revving prob?

hi i have a 98' ford ka. just started having a problem a couple of days ago. when im driving down the road if i take my foot off the accelerator or even just press it a little softer for a few secs, when i push the the throttle back down to accelerate the engine makes a really loud revving noise for a few secs then goes back to normal. only really notice it when im above 40 -50. makes a noise as if i was to push the clutch down while im still accelerating. my foot is no where near the clutch though. Any advice would be great, i dont know much about cars and dont want to get fleeced when i take it into the garage