Random variations in price and volume. These can distract a trader from the real trend. Noise becomes more noticeable when looking at shorter time frames.
One of the primary goals of the alerts server is filter out the noise and only report real trends. This is especially important because the alerts server makes use of tick data. If one trader buys 100 shares at an unusual price, that is not cause for alarm. If most of the trades are at a different price, we label that one print as noise, and ignore it.