SSE has been given six months by Highland Council to sort the problem at the station at the northern end of the new Beauly-Denny line or it will face legal action.
The sound emanating from the electrical base is deemed to constitute a "statutory nuisance" leading the council to issue a noise abatement notice under a law often used against noisy neighbours.
It follows complaints over many months from people living near the substation at the Beauly end of the 137-mile transmission line. They say the noise can be heard several miles away, and seriously affects everyday life.
Failure to comply with the terms of notice without reasonable excuse may result in prosecution in a sheriff court, under part three of the Environmental Protection Act 1990.
SSE says it is disappointed the Highland Council felt it necessary to threaten legal action, as it insists a significant amount of progress has been made and there is now a programme in place to resolve what was a complex issue.
However, Stephen Byford, chairman of Kilmorack Community Council, said: "It has been going on for a year now.
"We had already been suffering the effects of the Beauly-Denny upgrade and we shouldn't have had to put up with this as well."
He said one particular piece of equipment had been found to be responsible, a Static Var Compensator (SVC), a transformer that stabilised the voltage going down the line.
Repeated complaints had been made to SSE and the council.
Mr Byford said: "This thing hums creating a tonal noise which everybody in the area, even several miles away, can hear. I live a mile and a half away up on the braes above the substation and I regularly hear the noise, especially when you have a still night and are trying to sleep.
"All last summer we weren't able to sit in our gardens and relax because of the noise. There are about 1500 people here who are affected, but some in the neighbouring Kiltarlity parish are as well."
He said he had a colleague who lived three and a half miles away who could hear it occasionally.
Councillor Graham Phillips, chairman of Highland Council's Transport, Environmental and Community Services Committee, said the council appreciated the ongoing patience of the residents "as this complex investigation progresses to the next stage".
Alan Yates, Highland Council's environmental health manager, said: "Our officers have reached the stage of the investigation where they feel it is appropriate to serve an abatement notice to ensure the nuisance is resolved and to provide some reassurance to local residents."
He said the notice would give SSE six months, which was reasonable given the complexities of finding and implementing suitable solutions for a live substation of that size.
Alastair Brand, SSE's Beauly-Denny Project Director, said: "We are taking this matter extremely seriously and are disappointed that the Highland Council has felt it necessary to serve a noise abatement notice as we have been working closely with the council and the local community to address their concerns.
"We have made a significant amount of progress and there is now a programme in place to resolve the issue."
He said expert consultants had been appointed. SSE might also have to seek permission from National Grid to switch off some equipment in order to allow work to be carried out safely.