TAIPEI, Taiwan - The initial analysis of a blackbox from a doomed China Airlines jet has not immediately yielded any clues as to why the plane crashed last month killing 225 passengers and crew, but has picked up several unusual sounds, the chief investigator said Sunday.

Minutes before the Boeing 747-200 went down, the cockpit voice recorder picked up a noise that sounded like a human heart beat. But investigators have yet to identify the source of the noise, said Kay Yong, the chief investigator at Taiwan's Aviation Safety Council.

Shortly before the crash, the black box also recorded a noise that sounded like "ka ta, ka ta, ka ta," Yong said. The last noise was a sharp "thud" before the power went off to cut the recorder's tape, he said.

Several Boeing 747 pilots who listened to the tape said these were not sounds usually heard in the cockpit, Yong said.

Each sound lasts a fraction of a second. Investigators could not say if they were related to the crash, "but at this moment, we'd rather be more suspicious," Yong said.

A closer and sophisticated analysis was needed to identify the noises, he said.

"There's no way I can say when this analysis will be finished."

The second black box, the flight data recorder, was still being analyzed, and Yong would not comment on its contents.

Yong repeated on Sunday that the pilots' conversations did not indicate any problems.

He refused to speculate about why the plane crashed on May 25 about 20 minutes after taking off from Taiwan enroute for Hong Kong. But divers searching for the wreckage deep under the sea did not "find anything that did not belong to the airplane."

Search crews are still trying to recover large parts of the plane, which split into four pieces before plunging into the Taiwan Strait near the Penghu island chain, off Taiwan's western coast. The wreckage might offer the best clues about why the plane crashed.

Some aviation experts have suggested that metal fatigue might have caused the 22-year-old plane to break up. Others have suggested that an explosion — perhaps in the fuel tank — was to blame. Security officials have said there were no signs of terrorism or a missile attack.

Rescue teams have so far recovered 160 of the victims' bodies. Eight corpses — including that of co-pilot Hsieh Ya-hsiung — were found Saturday in wreckage about 60 meters (200 feet) under the sea.

The United Daily News paper quoted prosecutors as saying Hsieh's body was attached to the seat when it was found, indicating there had been no noticeable problem requiring him to get up immediately before the crash.