Loud metal sound when not pedaling I have a very simple city bike. I have neglected it outside for a while to a level I had to take it to a repair shop manually because of a blown tire. At the shop the tire was changed and the breaks readjusted.

But now when I am riding it I have a very loud cranking clanking sound (like the chain is hitting something, or something similar, don't know where it comes from) coming when I don't pedal for a few meters (for example going down a hill). It will go for a few more meters and stop, and then again. 
It barely happens when I am just leading the bike with my hands (it will continue if this sound started, and I got off the bike for a while, but then disappear), it also doesn't appear when I pedal.

I can't find the source because the sound is only on when I am on it and disappears when I am off it.

What could it be?

Thank you
If I understand well this noise only happens when the wheel is rolling and you are not paddling. If so, then the most obvious place to look first is the freewheel. It is in the back wheel axle.

You can try to reproduce the noize by lifting the back of the bike and spinning up the back wheel with the pedal to a high rotation.   

One cause could be the lack of grease/lubrication which can make the freewheel noisier.