Fork / Stem
Fork/Stem bearings allow for free rotation of the fork when changing direction. Therefore, damaged bearings can increase rolling resistance which will make the user spend more energy and stress while maneuvering.
A knocking noise, and on some occasions, a squeaky squealing moan is what would be the first indicator of warn bearings. Initially, the sound would be heard only at certain speeds, but will become fairly regular and prominent as the condition of the bearings worsen.