The lower pitch note may be hard to hear, it dies away more rapidly than the higher note. The higher pitch note goes on ringing for a long time, over a minute, it has a very pure sound. It has one frequency, the fundamental frequency of longitudinal motion of the bar. The note played by the bar is said to have a long sustain . With time, the organized motion of the atoms of the rod which we hear as sound turns into the disorganized motion of thermal energy, warming the rod imperceptibly.
Some energy goes into sound in the air.
Different materials have different rates at which organized motion is turned into heat. Aluminum bars ring for a long time. Steel bars ring for a shorter time and lead bars ring for a shorter time still. Hold the bar in the center and tap it downward near the end. Notice that the lower note is louder than before.