CONSTANT humming from part of a new £600 million power line is so loud it is waking nearby residents at night, it has been claimed.

Noise experts are carrying out tests near the newly expanded substation for the controversial Beauly-Denny power line after complaints from locals.

The small rural community around the Wester Balblair sub-station near Beauly claim they are unable to sit outside and are being woken up at night while a community leader said he had recorded the noise as being above acceptable levels.

The residents believe the noise is linked to the expansion of the sub-station at the start of the 137-mile overhead power line, which is being upgraded to 400kV to carry renewable energy generated in the north to consumers in the south. The first section of line between the sub-station and Fort Augustus went live two weeks ago.

Developer Scottish and Southern Energy (SSE) and Highland Council are investigating after residents noticed a recurring loud buzz.

Steve Byford, chairman of Kilmorack Community Council, said he had been woken several times in the early hours at his home in Broallan, about a mile-and-a-half away from the site.

On one occasion, he registered 43 decibels using a downloaded app. Department of the Environment guidelines suggest 30 decibels is an acceptable sound level in a bedroom at night.

"It is distracting and upsetting," he said. "The trouble is, with the nice weather when you want to open the windows and let in the breeze at night. The noise has been so loud you are not able to do that."

Night-shift worker Tara Ross, of Wester Balblair, described the situation as horrendous. "It is driving us crazy," she said. "I went to bed the other morning and it was a nightmare. I got about half-an-hour's sleep. You cannot sit outside in the garden on an evening. You are just zoned into the noise - you cannot block it out."

Cherry Ambrose said it was difficult to go back to sleep once she had noticed the noise. "I tend to switch on the TV so I can't hear it," she said. "I find it easier to go back to sleep with the sound of murmuring voices."

Gavin Steel, a liaison manager with SSE, confirmed complaints had been received from residents.

"We have arranged for a specialist consultant to carry out some noise monitoring," he said. "They are in the process of analysing the data and we expect to receive early results very soon."

In the meantime, he declined to speculate on the cause of the noise.