You should always alert crew if anything at all concerns you on an aircraft. The issue might already be known to crew, be unknown to crew or it may in fact be a completely normal part of operations that is unusual to the passenger. In any case, if in doubt, tell someone. As to the initial situation that started this thread.
1. Crew may not respond to a call button during taxi because they are required to be in their seat and secured at this time. In extenuating circumstances (and safety permitting) they may be released from their seat by whoever is in charge to ensure there is no emergency situation onboard.
2. The noise may be a known issue to the crew. They may have been operating on that aircraft for several flights and have already been alerted to the noise. The aircraft might also have been checked and cleared for operations since the issue became known.