A spokesman for the Australian Maritime Rescue Centre said: "It could be possible human tapping from inside the hull, yet could also be rigging just clanging around. But we are hopeful it is Tony Bullimore."
The news came as rescuers announced that the Frenchman Thierry Dubois - whose yacht foundered in the same area of Southern Ocean - had been lifted from his life-raft by an Australian naval rescue helicopter at 21.40 GMT last night. He was then transferred to the rescue vessel HMAS Adelaide where doctors were treating him for the effects of his ordeal.
Hopes that Bullimore was still alive rose after tapping sounds were heard via sonar buoys dropped near his yacht, Exide Challenger. A rescue vessel was due to arrive between midnight and 4am (GMT) this morning.
Rescuers believe the only way he could have survived the freezing conditions since capsizing on Sunday would have been to have put on an anti-immersion suit and taken refuge in the hull of the yacht.
Stephen Mulvaney, Bullimore's nephew, told Australia's Channel 9 television station that the news of the tapping sound was "very comforting" and he remained hopeful it was his uncle.
"It would be in line with what we have been hoping for. Tony is a very experienced seaman and he would take the action we think he has taken by staying inside the vessel," he said.
Bullimore and Dubois, competitors in the Vendee Globe round-the-world race, capsized on Sunday after hitting a storm 900 miles from Antarctica and 1,400 miles from the Australian coast.
Dubois was found by a plane a day later, clinging to the upturned hull of his yacht, Amnesty International, and managed to scramble into a life raft dropped from the air.
The catalogue of disaster threatened to mount further last night when race organisers revealed they had also lost touch with the Canadian Gerry Roufs, who had been lying second.
His 60-footer Groupe LG was in even more remote circumstances than Bullimore, 1,700 miles from Cape Horn, and being battered by winds reported to be gusting to 80mph.
The French solo sailor Isabelle Autissier, who was rescued two years ago by the Australian Navy when she sank in the BOC single-handed race and this time had to go to Cape Town for repairs, was asked to go to Roufs' assistance. She said conditions were so severe - she had capsized twice overnight - she did not think she could make it.
If Roufs is also in severe trouble it would mean that only seven of the original 16 on the start line at Les Sables d'Olonne in November are still officially in the race.Reuse content