So, the noise issue is outside the realm of believability, but Walkers aren't?
The suspension of disbelief requires that a series keep its own internal logic on a frequent basis. Zombies are part of the world of the show: my suspension of disbelief handles that. Daryl riding a loud motorcycle while everyone else is trying to tiptoe about and not make noise is simply ridiculous in the context of the show's own logic.
Rock beat me to it.
To me, the loud motorcycle is the equivalent of hearing the click of the safety as the glock is pointed or after a prolonged struggle followed by a monologue delivered at gun point, the guy with the gun decides he's going to rack the slide to show he means business.