[AnAmericanMother]

A cat yes, when they are looking out the window at a bird or squirrel and dreaming about catching them if they could just open that window.

That's quite a different sound, almost a glottal stop (can cats DO that, I wonder?) It's called "yammering" - that's where "Katzenjammer" comes from.

[Ditter]

We have had 5 Maine Coon Cats and family members have had them too, all of them have made a chattering, sort of clicking sound combined with a low meow, when looking at birds. It is not a purring sound. I don't recall hearing other cats, just MCC, making it.