A jet roared overhead, drowning out a developer's speech last summer at the Ashburn firehouse in eastern Loudoun County on his plans for a new community that would be built on 1,580 acres just east of that 19th-century hamlet.

For Vicki Gallant, an Ashburn resident, that jet was eloquent testimony to why the county should refuse to allow developers to build homes directly within the airport noise zone between their hamlet and Washington Dulles International Airport.

"We can handle that in a firehouse, but live under it? No," Gallant told the Loudoun Board of Supervisors this week when it considered the proposal for Ashburn Village, a planned 5,451-home community with 2.9 million square feet of commercial space.

Her testimony focused on a key issue -- airport noise -- that is dividing the county board as it grapples with the huge rezoning application. Ashburn Village is the first major development proposed for land northwest of Dulles Airport to come before the board.

How the board handles the noise issue is being watched closely by some key interest groups.

Residents in the area want guarantees that they, and future residents, will be protected from noise pollution.

The Federal Aviation Administration (FAA) has similar interests. Already burdened with noise complaints from airport neighbors nationwide and facing mounting pressure to close down such noisy airports as Washington National and those in Denver and Los Angeles, FAA officials would prefer that county planners save them one more headache by banning residential development from noise zones around Dulles Airport, said Henry L. Mahns, FAA's planning manager.

Developers, on the other hand, say the noise zones for Dulles Airport drawn by FAA and county planners cast too wide a net over their land, wiping hundreds of acres of land suitable for residential developments off the map.

"It's so ludicrous as to be insensible," said Rosser H. Payne, a planner for the Ashburn Village development. He said Loudoun is trying to defy the rising tide of development in the county by using the noise issue.

Each of the differing interest groups views the Loudoun board's vote on the Ashburn Village proposal as a test case for its cause. They said the board's decision will set a precedent for how strictly the supervisors will adhere to a noise zone designated in its recently adopted planning guidelines for the land north of Dulles Airport.

In the county's planning documents for the area, adopted last month by a 5-to-3 vote, the board agreed to ban new residential development from areas with average noise readings above 65 ldn (loudness day night), a noise measure that averages the peaks and valleys of decibel readings from jet noise, giving extra weight to nighttime noise, which is considered a greater irritant.

Loudoun County has used the most recent FAA study to decide where to draw its 65 ldn line. The federal government is expected to adopt this study next year.

The problem is that the 65 ldn line in the FAA study bisects the proposed Ashburn Village development and, if strictly adhered to, would force developers to abandon plans for construction of 1,200 homes on 300 acres. That would cripple the project, Payne said. Ashburn Village is a joint development of Associated Investments Co. Inc. and Richmarr Construction Corp.

So the developers are contesting the methodology that Peat Marwick Mitchell & Co., a consulting firm, used to develop the 65 ldn line.

This has plunged the supervisors into a bog of technical arguments over such things as changes in jet technology, assumptions entered into computer models to predict future growth at Dulles, flight paths and take-off and landing procedures.

Payne has argued that the noise projections beyond the year 2000 for Dulles Airport assume the airport will surpass Washington National in size and become one of the busiest airports in the world, which he said is an unreasonable assumption.

The FAA's Mahns said Dulles had 542 flights arriving and leaving daily from its three runways in September; the FAA projects that by the year 2000, when five runways are in operation, the number of flights daily will almost double to 1,079, serving 14.2 million passengers.

Payne contended that air traffic could be redirected away from the Ashburn project. Confident that the 65 ldn line can be shrunk if various factors are taken into account, the developers have offered to pay $15,000 for their own consultant to update the FAA study.

But Ronald Tulis, a Peat Marwick official, is standing by his figures.

"What they're asking for is for us to put all the aircraft noise over somebody else's property. That just doesn't happen in a democracy. We can't bend over backwards to help out one developer," Tulis said.

The county board is scheduled to vote on the Ashburn Village proposal Dec. 2.