A series of pings detected in the southern Indian Ocean and originally believed to have come from missing Malaysia Airlines jet MH370 are now thought to have been emitted from either the searching ship itself or equipment used to detect the pings, a US Navy official says.

Michael Dean, the US Navy's director of ocean engineering, told CNN that authorities now believed the four acoustic pings at the centre of the search off the West Australian coast did not come from the missing passenger jet's black boxes, but from a "man-made source".

Specialists prepare to deploy the towed pinger locator on the Australian vessel Ocean Shield. Credit:Reuters

"Our best theory at this point is that (the pings were) likely some sound produced by the ship ... or within the electronics of the Towed Pinger Locator," Mr Dean told CNN on Wednesday.

"Always your fear any time you put electronic equipment in the water is that if any water gets in and grounds or shorts something out, that you could start producing sound."