Radar can be heard purring from across the other side of the room. He has several different happy purrs for different situations.
For example: this morning when I first went into the sitting room he launched himself onto my chest and went into a headbutting and kneading frenzy, he was wriggling around in my arms 'kissing' my face and giving me head rubs and full body rubs all over me. This was accompanied by a very deep, fast, rumbling purr, with little grunts and chirps, it sounded almost frantically joyful. It makes me feel very loved when he does this
When he is dozing on my lap he behaves more relaxed, likes to lie with his head tucked under my elbow and his back resting along my arm. His purr then is much softer and slower, very relaxed sounding. It tapers off as he drifts to sleep, and starts up again if I move slightly or stroke him.
When I am cleaning his bowls and feeding him, he purrs quietly but quite fast. It sounds happy but with a 'come on hurry up and give me my dinner' tone to it
And lastly, he purrs when he is picked up and carried. This is mid-way between the dozing off on my lap purr, and the being fed purr. Quite quiet and soft, and between the two in speed.
Thankfully I have never heard him purr due to discomfort or pain, I hope never to have to.