I've had a cat for ages and am not quite sure what exact sound they are referring to? I've certainly heard a meow crossed with a purr sound, which seems a bit like what they describe, but it's not when she wants food; she does that if you wake her up by suprise (she's old and quite deaf now), or just as a sort of "hello". She actually can't do a normal low purr anymore though 
Usually just a normal meow if she wants food I find, no purring element, always has been

Usually just a normal meow if she wants food I find, no purring element, always has been
