My dragons have a... "difficult" personality.

They're very catlike. Sure, they're more like KrimsonRogue's cat (somewhere around chaotic good). But, if I want to make my dragons smug, blue-blood a-holes that you still want to cuddle, then I might as well give them the ability to purr very loudly.

Well, that actually would have one tangible advantage, if the dragon could use it to transmit data and instructions to the nanomachines in their body.

Problem is, dragons here are about as large as a draft horse, are filled with spongy and hollow structures on top of having air sacs to assist breathing, and I'm afraid that would ruin the acoustics.

Basically, there are two unique features to my dragons:

 1. Air blubbers, a special type of aerogel, made out of fibrous, CNT-reinforced organic matter with densities around 200 kg/m^3.  
 These air blubbers are primarily located under the dragon's skin, serving as protection against blunt force trauma (like crash-landing).
 2. Air sacs and hollow bones are present throughout the body, as dragons have a respiratory system similar to birds'.

There's also that large cats (lions, tigers, etc...) can't purr.

**So, how would dragons be able to purr very loudly?**