1. A diagnostic apparatus for detecting a loose part moving in a conduit carrying a flowing material, comprising: an acoustic pickup operable to produce an electrical signal in response to vibration of the conduit, including vibration caused by the loose part and background vibration; a signal processing means coupled to the electrical signal of the acoustic pickup, the signal processing means being operable to encode a plurality of values representing distinct parameters of the electrical signal, in an ongoing manner, the signal processing means providing a plurality of discrete outputs representing values of the distinct parameters; a neural network having a plurality of neuron cells, each defining at least one weighting factor, the neuron cells having inputs coupled to the values of the distinct parameters and being operable to weight the values of the distinct parameters to produce a neuron cell output; and, at least one summing cell producing a sum of a product of the weighting factors and the values from each of the neuron cells, and wherein the weighting factors complement at least one combination of the distinct parameters defining a signature of the loose part, whereby upon passage of the loose part the summing cell produces an output that is substantially displaced from a level resulting from the background noise.