HEAR that? Someone in the flat above is playing with marbles. But it's 1.30am. No child would be up at this witching hour.

The mystery deepens for bank executive Joseph Wong, 47, because his upstairs neighbours are an elderly couple. No youngster live in their five-room HDB flat. (Our 1st Forum Exchange)

When The Sunday Times interviewed the families living up to three floors above him, all denied dropping marbles or any other objects on the floor so late at night.

Also, the sound seemed to have followed Mr Wong from Lorong Ah Soo to his new home in Edgefield Plains in Punggol.

Wags, of course, would say he has lost his marbles.

But his letter to The Straits Times Forum Page last week, asking the HDB for answers, has stirred out of the shadows many who have been similarly vexed, if not puzzled.

It has also plunged Singaporeans into a frenzy of fuzzy theories. The Straits Times received more than a dozen letters, including one from Madam Sharini Shamsuddin, who playfully suggested that spirits are at work.

Internet forums are swelling with explanations that make you want to throw up your hands in disbelief. One that probably coaxed many laughs, both nervous and mocking, is the suggestion that the HDB had placed marbles between floors so that playful spirits would be so distracted playing with them, residents will be left in peace.

If so, private developers have been equally diabolical. Expatriate Julian Cohen, 42, has been haunted by the same sounds at each of the three private condominiums he moved into in the past 10 years. He also heard them when he was working in Hong Kong.

He said wryly: 'I guess Joseph can take consolation that the marble-dropper doesn't just live above his HDB flat but drives round to his second apartment above mine to continue his marble-dropping activities after he has woken Joseph up.'

Mr Wong's wife Geralda and six-year-old daughter Amanda, however, are sound sleepers, deaf to the disturbing noise. 'Always, the noise happens between 1.30am and 4am. It spooked me initially but as the years go by, you get used to it,' he said.

Postgraduate student Lim Leng Hiong, 28, who lives in Holland Drive, suspects golf enthusiasts are practising their putting at home.

In an experiment, he found that glass marbles, ping pong balls and golf balls, when dropped, produce different sounds. 'But it's hard to tell the difference when heard on their own,' he noted.

The only problem with his explanation, of course, is that HDB dwellers have been hearing the sounds of marbles way before the recent golf craze.

HDB would have you believe that every sound has a reason. It said in a letter to The Straits Times: 'To date, we have not received any feedback on the sound of dropping marbles where there has not been any attributable cause.'

What could be some of the attributable causes? HDB spokesman Tay Boon Sun is not telling. But he gave the assurance that the acoustic properties of flats do not differ from other residential buildings.

More forthcoming are structural engineers and the Founder of Asia Paranormal Investigators, Mr Charles Goh. Their answers, however, indicate that when things go bump in the night, the reasons are rarely beyond the mundane.

Mr Goh, 37, said the marble sounds can be traced to air-con pipes or old refrigerators, while the sounds of furniture being dragged across the ceiling, another common occurrence, could be due to lift doors opening and closing.

Structural engineers point to two probable causes: the plumbing and the concrete walls and floors.

Mr Wong subscribes to the last because that's where he hears the marble sounds in his flat.

Mr Crispin Casimir, owner of CC Building Surveyors, and Dr Tan Guan, a director at TY Lin SEA, will have you focus on the concrete.

Concrete floors may contract at night, giving off creaking sounds, Dr Tan said.

Mr Casimir added: 'Sounds travel and are distorted as they pass through concrete walls and floors.'

Mr Shek Kam Chew, 68, a structural engineer since 1966, is convinced it's in the plumbing.

'It's definitely not supernatural but what is called 'water hammer',' he said.

Water hammer occurs when water flow is shut off suddenly like when the toilet is flushed. This change in pressure rattles the piping and create 'clanging'sounds, which may sound like marbles dropping after distortion through concrete walls, he said.

Still sceptical? Why not stick with the obvious? Surveys have shown that Singaporeans sleep late. So really, it's just children playing with marbles.

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Three explanations

1 When water flow is shut off suddenly, the change in pressure rattles the pipes, which may sound like marbles dropping after distortion through walls.

2 Some pre-fabricated floors used in HDB flats have an embedded high-strength steel cable which can give off creaking sounds when they contract.

3 It is really children playing with marbles

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What the HDB says

'HDB flats are designed based on prevailing building codes. Hence, the acoustic properties for flats are no different from other residential buildings. To date, we have not received any feedback on the sound of dropping marbles where there has not been any attributable cause. HDB Sengkang Branch Office, which manages the flats at Punggol Town, has not received any feedback about the sound of dropping marbles in the area.

In Mr Wong's case, we have contacted him and offered HDB's help. We will be checking with his neighbours over the next few days to see if we can locate the source of the sounds Mr Wong mentioned. Madam Shamsuddin mentioned that her eldest son described hearing similar sounds as Mr Wong. If she would like us to look into the matter too, she can contact our Toll-free Branch Office Service Line: 1800-2255-432 to provide us with her address.

In a high-rise, high-density living environment, a certain degree of noise is inevitable. It would also be in the interest of neighbourliness for residents to ensure that the activities carried out within the flat do not generate excessive noise and to exercise some degree of tolerance towards their neighbours.'

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What the readers say

'The pipes of some air-con units make popping sounds. These sounds resemble marbles dropping when they travel through the pipe embedded in walls. Another possibility is that in some old fridges, the compressor makes this noise which can be heard clearly only at wee hours when it's quiet.'
-- MR CHARLES GOH, Founder of the Asia Paranormal Investigators

'I'm quite sure that this 'urban legend' is due to a real social phenomenon: the Singaporeans' love of mahjong.'
-- MR CHRISTOPHER YEO JER SIONG, Chuan Terrace

'Could the sound be attributed to the expansion of a certain type of tiled floor in the much cooler early hours of the morning?'

Charles Goh: Finally the marble dropping phenomena reached its climax when the Straits Times published an article citing the results of an investigation by a paranormal group, SPI, on June 26th. ........