A scene in Season 5’s second episode shows Drogon reuniting with Dany and making a purring noise that was created by Fairfield recording her dog, whose happy whine was slowed down to sound more dragon-like, thus creating Drogon’s purr.
For instance, Paula Fairfield portrays Drogon’s love for Dany deeper than the other dragons, and has used personal connections from her own life to help that chemistry show.
The scene in question can be found below, and with the knowledge that it was a dog, and not a real-life dragon (naturally), that was being recorded.