Let $n=15r$ where $r>5$. If $r\!\!\! \mod 15 \equiv w$ then is it true that $\Phi_{n}(x)$ is not flat whenever $2<w<13$ ? In other words, are the flat ones necessarily among those satisfying $r\equiv \pm 1 \!\!\! \mod 15$ and $r\equiv \pm 2 \!\!\! \mod 15$