Its probably a float to int rounding problem. Try using floor(y) when drawing tiles, see if that helps - it'll force everything to round down to the nearest int, rather than just going to the nearest whole number.
[edit] Alternatively, calculate delta * scrollingspeed only once, then add the returned value to all your y coordinates, rather than calculating the whole lot for every individual tile (this'll give you a minor speed increase anyway)
[edit] Alternatively, calculate delta * scrollingspeed only once, then add the returned value to all your y coordinates, rather than calculating the whole lot for every individual tile (this'll give you a minor speed increase anyway)