Not only do they have frequencies, but the range of frequencies LIGO detects are partly within the limits of human hearing.
So you can listen to [natural-pitch black hole merging noises.](https://www.youtube.com/watch?v=AG19o2PmnNo
The frequency they produce is related to how fast the holes are orbiting eachother. As they spiral towards eachother and shed energy as gravitational waves, they orbit faster and the frequency goes up. So if you adjust the pitch they sort of make slide whistle noises.