Active Sound Generator

Guys, does anybody know how to make an "Active Sound Generator"?

It produces an amplified engine sound and the car sounds louder. In retail it costs about 2000€ that's why I would like to built my own Active Sound Generator.

Any car fans who can help me out?

Get the sound you like, put it on a small MP3 player and adjust the desired noise level with an amp suitable for the speakers in question.
But you do realise that all it does (apart from a fine by the cops) is to make people laugh at you once you open the bonnet or accelerate ? ;)