Representatives of a South Africanbacked interim government in Namibia, arriving here to lobby for a shift in U.S. policy toward that area, said yesterday that there are "vibrations" of encouragement from the Reagan administration.

Dirk F. Mudge, chairman of the Democratic Turnhalle Alliance, said he and two other Namibian representatives disregarded a State Department plea to defer their trip because "to come after they formulate the policy would be too late."

Mudge said his delegation would not see senior State Department officials, due to the unresolved state of U.S. policy on Namibia's future. They are scheduled to visit eight U.S. Senators, including Charles H. Percy (R.-Ill.), chairman of the Senate Foreign Relations Committee, and Jesse A. Helms (R-N.C.).

The Carter administration was a key backer of a United Nations plan for a cease-fire and elections in Namibia, a mineral-rich former German colony that has been run by South Africa since World War I. South Africa and the Turnhalle Alliance refused to accept the plan early this year, leading to collapse of a U.N. conference on Namibia's future.

The other leading contender for power in Nambia is the South-West Africa People's Organization (SWAPO), which has led a guerrilla war and is backed by most black African states and the United Nations.

Mudge charged yesterday that SWAPO is indebted to the Soviet Union for military and political support and will have to repay this debt, perhaps with mineral rights, if it gains power.