If you've installed a sound pack for GPL, the game will read the engine sounds from a wav file in the sound directory.
Being the silly guy I am, I opened sound recorder and recorded me humming, and then making a throaty engine sound, saved it as a Ferrari engine sound (fer.wav). On taking it for a spin, the humming sounds like a bee at 10,000 rpm, and the throaty sound is a bit, erm, weird.
The next step on the sillyness was getting a fart sound and putting that in the game. Sadly it doesn't quite work as well as you might think.
Still worth a try though