Jet engines are generally louder than the sound produced by a Harley-Davidson motorcycle or the Republic Thunder screech, which is not an actual aircraft.
Jet engines produce a significant amount of noise due to the high-speed exhaust gases and the turbulent airflow generated by the engine's operation. The noise level of a jet engine can vary depending on factors such as the type of engine, the aircraft it is installed on, and the distance from the source of the noise.
Harley-Davidson motorcycles, while known for their rumbling exhaust note, typically do not produce noise levels as high as jet engines. However, the sound of a motorcycle can still be subjectively loud, especially at close range or when the engine is revved aggressively.
As for the "Republic Thunder screech," there is no such recognized aircraft by that name. It might be a misinterpretation or a fictional reference. The loudest aircraft in history, often mentioned, is the Soviet-made Tupolev Tu-95 "Bear" bomber, known for its distinctive loud engines. However, it is worth noting that noise levels can vary depending on the specific operating conditions and the observer's location.
In summary, jet engines are generally louder than Harley-Davidson motorcycles, and there is no verifiable information about a "Republic Thunder screech" aircraft.