on my mk6 escort. (1.6 zetec) i ws driving it around, i think its the first time for a week and anyway it was making a whooo whooo noise so i pulled over to see if a back window was open.

when i pulled over the revs started going from 500 to 1500 RPM up and down about 6 times so i revved the engine to abut 4000 RPM held it there for about 3 seconds and let off then it was fine..........

can anyone tell me why it did this?

thanks for help