No, a Harley-Davidson motorcycle is not as loud as an airliner. The perception of loudness can be subjective, but in terms of objective measurements, the sound produced by an airliner at full power is significantly louder than that of a Harley-Davidson motorcycle.
Airliners produce a high level of noise due to their powerful jet engines, which generate thrust by expelling a large volume of air at high speeds. The noise from an airliner's engines can reach levels of around 140 to 150 decibels (dB) at close range, which is well above the threshold of pain for the human ear.
On the other hand, Harley-Davidson motorcycles, while often associated with a distinctive and loud exhaust note, typically produce sound levels in the range of 80 to 100 dB when measured from a distance of 25 feet (about 7.6 meters). This is considerably lower than the sound levels produced by an airliner.
The perception that a Harley-Davidson motorcycle may seem louder than an airliner could be due to a few factors. Firstly, motorcycles are usually encountered at much closer distances than airliners, allowing the sound to be more noticeable. Additionally, the unique rumbling exhaust sound of a Harley-Davidson motorcycle may create the impression of a louder noise. However, if you were to measure the actual sound levels objectively, an airliner at full power would be significantly louder than a Harley-Davidson motorcycle.