A second mystery explosion has been reported in the Hinterland with police speculating chlorine bombs could be rocking the area.

The latest explosion heard about 8.30pm on Wednesday has again baffled police, the air force and seismology experts who didn't register any earth tremors from this or the first explosion.

"If residents felt it then we should have registered something," said a spokeswoman from the Seismology Research Centre in Brisbane.

"The only explanation I can think of is the explosion didn't come from the ground."

Guanaba residents Ron and Donna Dimmick were eating dinner when they heard the latest explosion.

"It was so loud, just a loud boom and then nothing. It was huge and quite loud, it was a strange explosion," said Mrs Dimmick.

"I was in the earthquakes in Newcastle and the bang was just like the climax of the earthquake except nothing shook here."

Most residents said the explosion sounded a little further away than the one the previous Thursday but this time there were no reports of houses shaking.

The air force said it had no planes in the area on Wednesday night but back-flipped on their denials of any planes last Thursday saying they had two F-111s around Canungra that night.

"Air force can confirm two F-111 aircraft from 82Wing located at RAAF Amberley were engaged in night flying training over the Evans Head Air Weapons Range on the evening of Thursday, February 5," said a spokeswoman.

"At the completion of the training activities the aircraft returned to RAAF Amberley at an altitude of approx 12,000ft at 450 knots. The route taken would have passed approximately 40km to the west of Canungra but at no stage did either aircraft approach or exceed the sound barrier."

A sonic boom is caused when the sound barrier is broken and sends out a 'pulse' that has the force to cause houses to shake.

Al Bressow and Jodi Gow, of Guanaba, said the first explosion had scared all their horses to the top of their paddock while Wednesday night's explosion sent a pulse through the house.

"We could feel something go through the house and it sounds like it's only 50m away," said Mr Bressow.

"But when we talked to our neighbours they thought it was us but we thought it came from their direction.

"It wasn't a plane hitting the sound barrier, it sounds more like something exploding in a 44-gallon drum.

"It's too loud to be below the ground."

Guanaba resident Jim Ingram heard both explosions and said Wednesday's explosion was 'exactly the same' as last week.

"It was a strange experience, I didn't know what was happening," he said.

"It was an absolutely huge noise and the strange part was the dogs didn't bark at all and they bark at anything."

Acting Superintendent Chris Emzin said police were probing both explosions.

"Initially we thought it may have been a natural phenomenon but given that it has happened a week later, we suspect it could be misuse of explosives or firearms," said Supt Emzin.

"It's a rural area and the noise is carrying to large numbers of residents in the area.

"From our point of view, we'd like to remind people that any misuse of firearms or explosives is very dangerous and it also renders people liable to serious penalties.

"They (the explosions) appear not to have been in the exact same spot, but within 2km of each other.

"There is an industrial area (at Guanaba); there have been inquiries made there and there doesn't appear to have been any explosions of any generators or transformers there.

"Certainly it's a loud enough explosion for people to have concerns about it. It could in fact be a chlorine bomb, given the density of the area and the potential for the noise to reverberate around the area."

A chlorine bomb is made by mixing alcohol and chlorine in a bottle, sealing it and then shaking it before putting it down. After a minute or two the bottle will explode.

Investigations into the explosions are continuing.