I would think that they're referring more to the rumbling sound of a small two-stroke engine idling as opposed to a chainsaw running at full throttle while cutt

I would think that they're referring more to the rumbling sound of a small two-stroke engine idling as opposed to a chainsaw running at full throttle while cutting wood. Here in the southeast US, at least, it not unusual to describe the sound of an engine idling as "purring" or even "purring like a kitten."