I've herd multiple (pun intended).
Whisper, I've only seen rarely. Clowder almost as rarely.
The most common I've run into is a Pride, when folks aren't just using a generic collective like herd or group.
But I have heard whisper used, though it was most often in a regular voice; neither whispered nor shouted. But sometimes it was whispered. Whispering about a whisper of whiskered cats just seems appropriate to me.
Imagine, you wake up and see a thousand cats surrounding your house. They sit, silently staring at your door. Wouldn't you want someone to come up behind you and whisper into your ear "don't open the door or that whisper will find its way in, and silently siphon our souls"? And then you could ask "why are you whispering, and how did you get in my house?"
And, of course, you would turn and there would be nobody there, only the fading outline of someone you can't quite remember because their soul was taken already, and memory drifts away when one is stolen.