I got to go for a little ride after work when it got up to 50 degrees today, wanted to see how the db killers sounded installed in the urban brawlers, noticed a whirring sound as i was riding , that corasponded to acceleration, mind you it was slow riding just mainly going through the gears at around 35oo rpms, the sound increased with the throttle while moving, but in neutral just reving you dont notice it, im assuming it is intake noise, kind of reminds you of the super charger sound, on a side note the urban brawlers with the db killers are perfect nice sound not to loud and pretty quite cruising at around 60 mph. I guess comming from a twin these 4 cylinder noises are new to me.