I had a new Shimano 105 cassette and chain put on my bike by bike shop. When I exceed 25 mph without pedaling, I get loud screeching sound (and feel some gripping), when I pedal it clearly goes away. What can it be? My brakes are not touching my wheels. I think it is my cassette but am open to any other ideas.
If it doesn't happen when you pedal ( and it wasn't like this before you took it to the shop ), it sounds like someone who worked on your bike at the shop managed to displace the plastic cassette guard that sits between the cassette on the spokes, or they somehow negatively affected the rear wheel hub. At any rate it, they didn't reassemble your bike properly. If I were you I would take it back to that shop right quick and make sure they fix it for you for free.