Bicycle Tutor

I have a Corratec MTB. Everything was fine, but 2 months ago i took it to bike shop and changed its tires. Since that day i have a very strange problem.

Sorry but English isnt my native language, so i will do my best.

Lets say im pedaling, and i suddenly stop pedaling and start floating. A sharp sound comes from rear of bike. A continous sharp sound as if something is rubbing. If i start pedaling at this time, the sound goes away. However if i dont, the sound keeps until the bike stops.

1) It does this sound only on uneven surface. It doesnt do it on asphalt.
2) It does not always do it. Even if i start floating on uneven surface it sometimes does it.
3) I have disk brakes. The bike shop tried to understand the sound, he even disattached the brakes but the sound was still there.

Thanks

Sorry, floating should be "coasting"

I know this post is like 7 months old, did the bike shop look at the cassette to make sure it wasn't something not installed correctly?