Whenever he's quiet, there's a good chance he's doing something bad.
Whenever he's not making any noise, he's probably doing something naughty.
この場合、次のように言うことができます。
ーWhenever he's quiet, there's a good chance he's doing something bad.
「静かな時はいつも何か悪いことをしている可能性が高い」
there's a good chance ... で「…な可能性が高い・十分あり得る」
ーWhenever he's not making any noise, he's probably doing something naughty.
「物音を立てていない時は多分何か悪いことをしている」
naughty で「いたずらな・わんぱくな」
ご参考まで!