The rebuilding of Wolf Trap Farm Park's Filene Center, destroyed by fire last spring, is expected to start late this year after the Senate, as its last act early yesterday, approved a package of loans and grants to cover the cost of a new $17 million, 3,500-seat amphitheater.

The Wolf Trap bill, approved by the House of Representatives last month, was passed in the final frantic hours before Congress recessed after last-minute maneuvering by Rep. Frank R. Wolf (R-Va.), Sen. John W. Warner (R-Va.) and a group of bipartisan supporters.

"If it had not passed, we would not have been able to rebuild Wolf Trap in time for next year," said Wolf.

"The goal is to open up in August 1983, and had we had another two months' delay until Congress returns from its election recess we might not have been in time."

Warner called the bill "a fitting finale" to the session.

"We are deeply appreciative and very happy," said Wolf Trap Foundation President Carol Harford, who said she expects construction on the new Filene Center, modeled after the old one, to start Dec. 1. "It's going to look to all of us like the old Filene Center we used to know and love."

The bill authorized an outright $9 million federal grant for the construction of the new theater, to be built on the site where the original Filene Center burned to the ground on April 4. The 117-acre Wolf Trap Farm Park, founded by Catherine Filene Shouse, is the nation's only national park for the performing arts.

Also included in the bill is an $8 million loan, to be repaid to the federal government by contributions raised by the private Wolf Trap Foundation. Harford said yesterday the foundation has already raised $1.7 million from more than 9,800 contributors. "But obviously, we have a long way to go," she said.

In addition, the bill requires the foundation to take out insurance--at an estimated $400,000 a year--on the new building.

The old theater, built 11 years ago, had not been protected by fire insurance under a federal policy that "self-insures" government-owned buildings.

Congress has also required that sound barriers be erected to protect the theater and neighboring residential areas from traffic noise of proposed commuter lanes to be built alongside the Dulles access road.

The new lanes are expected to be completed in two years.

The bill for Wolf Trap had been held up in the Senate during recent weeks. The legislation was finally pried loose in the Senate when Warner and other supporters won approval for the House-passed version, then returned it to the House for final consent as the session was about to end.