A vacuum leak is most easily described as the unintended introduction of unmetered air ibnto an engine’s intake, by way of one or more compromised hoses, gaskets, or components. This unmetered air is problematic, due to its tendency to … in certain cases, a vacuum leak can actually be heard, thereby expediting the diagnostic process. However, as a general rule, substantial leaks are much easier to locate in this manner, than leaks of relative minor severity.