1. Apparatus for detecting bleeding at an internal site using ultrasound, comprising: (a) an ultrasound transducer (b) a control system coupled to the ultrasound transducer to control its operation; and (c) a tissue vibration processor that processes the ultrasound to identify tissue vibrations caused by internal bleeding, producing a signal indicating the internal bleeding, wherein the tissue vibration processor configured to identify the tissue vibrations, producing a tissue vibration signal, and filters the tissue vibration signal, producing a filtered signal from which any contribution to the tissue vibration from a source other than bleeding at the internal site has been substantially minimized, and wherein the tissue vibration processor configured to identify tissue vibrations at the internal site by: (a) estimating a correlation matrix from the color-flow signal; (b) carrying out an eigen decomposition of the correlation matrix to identify a signal subspace and a noise subspace; (c) estimating a frequency of dominant vibration components in the signal subspace and the noise subspace; and (a) based upon an estimate of the frequency of the dominant vibration components, determining a vibration amplitude estimate and a vibration frequency estimate, at least one of the vibration amplitude estimate and the vibration frequency estimate comprising the tissue vibration signal.