For the last five weeks, when residents in several Montgomery County subdivisions have turned on their faucets, the tap water that gushed forth has been dirty and discolored.

"It wasn't just yellow," remarked Alice Sasthoff,a Bethesda resident, of the water that came from her tap one day last week. "It was muddy. There were flecks of mud in it. My washcloth looked like I had bathed in Rock Creek."

Ethel Meisels of Chevy Chase washed a lod of white clothes at home one day last week. The clothes came out of the tub "muddy brown. The water was full of little brown specks," she said.

The discolored water, which has occurred and then vanished sporadically in such widely separated areas as Potomac and Gaithersburg, has forced some county residents to stop washing their laundry at home to buy bottled water for drinking, and take baths in friends' homes in other parts of the county.

Yesterday officials of the Washington Surburban Sanitary Commission, which is responsible for maintaining the county's water and sewer pipes, said they think the discoloration is an aftereffect of the unusually cold winter.

They said the discoloration now seems to have largely vanished and telephone complaints have dropped sharply from the average of 100 a day the WSSC was logging several weeks ago. The agency received about 2,500 complaints altogether, officials said.

Richard Hocevar, the WSSC's director of maintenance said the discoloration was caused by iron water pipe filings that normally collect on the bottom and sides of the water pipes.

Hocevar, who directed the agency's investigation of the discolored water, said WSSC officials think it was caused by the combination of a sudden 10-degree rise in February of the water temperature of the Potomac River, which supplies the county's water, and a thawingof the frozen soil around the pipes as well as a thawing of frost within the pipes themselves.

Hocevar said the three actions combined to increase the velocity of water rushing through the pipes, which scoured the sides and bottom of the pipes and shook loose the sediment, sending it flowing into homes.

He reported that the agency is bringing in two water systems experts from Illinois and Florida to review the WSSC's findings. Hocevar emphasized the small fine particles present no health hazard.

"We've never experienced this problem before," said commission chairman Johanna S. Norris yesterday after discussing the matter briefly with the Montgomery County Council. "It would be well for us to learn what to do to prevent (its occurring) in the future."

Agency officials were baffled by the seemingly capricious appearance and disappearance of the discolored water in different parts of the county. Hocevar said that several times when crews were sent out to check complaints, the discolored water had become clear by the time the crew reached the caller's home

Alfred Machis, director of the bicountry agency's operations and research branch, said no one at the agency remembered this sort of problem having occurred before.

"Usually, some discoloration will result from firemen having to use a large amount of water to put out a fire or from a broke water main," Machis said. "But that affects only a small area. These widespread reports of discoloration were completely new to us."

Machis said Prince George's County did not experience a similar problem because its water supply is taken from the Patuxent River, which is usually warmer than the Potomac, and because its water is taken directly from a reservoir where the water is not as affected by temperature variations.

Hocevar said WSSC crews since later last week have been opening up hydrants that are supplied by major water trunk lines in an effort to flush the discolored water out of the system.