The phrase "deep growl" refers to a low, rumbling sound. It typically describes the sound made by animals, like a lion or a dog, when they are angry or threatening. Full definition
The intensity of that growl changes depending on engine: from the subtle, slightly frantic sound of the 4 - cylinder, to the metallic rasp of the V6, to the satisfyingly deep growl of the V8. (kbb.com)
For the first, oh, thirty seconds, it sounds like there's an emissions pump running, and all you hear is a tremendously deep growl overlaid with the turbo's whistle. (automobilemag.com)
Installing a huge freeflow exhaust and the biggest rear silencer box you can fit will give you a nice deep growl. (mechanics.stackexchange.com)