ABSTRACT: This letter reports results from time domain measurements in a terahertz pulsed imaging system to demonstrate that a mechanical resetting mechanism in the pump-probe delay stage results in a small but resolvable noise signal. In the setup described here, this effect dominates all other sources of noise such as the background Johnson noise or shot noise, and can hence be isolated and analysed in detail. An analysis of the noise signal is used to estimate the physical limitations of the pump-probe system being employed. A comparison of the results with an analytic prediction allows us to formulate a useful and general signature of registration jitter, that should make it easy to detect in any sufficiently narrowband signal.