Why are Harley Davidson Motorcycles So Loud? [video] There’s no mistaking the sound of an approaching Harley-Davidson, yet what makes these bikes so loud? That’s the question Today I Found Out addresses in this episode. No earplugs required. Straight off the line today, Harley exhaust systems can emit noise as high as 80 decibels (dBs), give or take depending on the model. For reference, a typical car when idling emits noise at about 35-45 dB and in the high 50s to low 60s decibel levels at freeway speeds. Even something like a 2008 Ford F-450 truck only gets as loud as 51 dB when idling and 68 dB when traveling at 65 mph.