The ESS has a nice rumble. A very nice rumble. It's quiet though. You don't get any drone or any rasp and unless you really get on it, it's pretty subtle. For someone who wants to fly under the radar, it fits the bill perfectly! Had mine installed for months now and all I can say is, I'm very happy with it! Also, I like to call the ESS the 'mini Akra'. In terms of tone, it sounds just like but tends to scream a bit more towards redline. It's half the price of the Akra, it gives you about half the performance gain of the Akra, and you lose about half the weight of the akra, and you pay HALF of the Akra!! I have been enjoying mine!
Did I mention no CEL? That was icing on the cake for me!
Look at this thread:
http://www.m3post.com/forums/showthread.php?t=872102