Isolated black holes (BHs) don't emit gravity waves. A pair of BHs (binary BHs) do in the final stages of coalescence (merger). Some supermassive black holes found in the centers of galaxies are actually a binary pair of SMBH. It takes a Hubble scale length of time (billions of years) before these two massive objects get really close to each other. Then within seconds the pair merge, releasing gravitational waves.
res ipsa loquitur
@TheAstroChuck Leonardo considered English la lingua pura because, "It was beyond the corrupting forces of the Vatican". so now this is the opposite ,
good to hear your comments, though sometimes think it is pearls before swine
How can they not be emitting gravity waves when they are pulling in stars and the pair of BHs eventually merge? Sounds like gravity!