The Japanese government, which this spring agreed to restrict auto exports to the United States, has asked U.S. officials to consider lowering the tariff on Japanese truck cab chassis entering U.S. markets, U.S. Trade Representative William E. Brock said yesterday.

During a briefing with reporters following a 31-day tour of the Pacific, Brock said he told Japanese government officials that he will consider a tariff reduction, but since high interest rates are stalling automobile sales in this country, "It's a particularly sensitive time."

Tariffs on Japanese truck chassis were raised from 4 percent to their current level of 25 percent about a year ago.

Brock also said he doesn't "see any change in the basic fiscal or monetary policy" of the Reagan administration that will lower interest rates. "I don't think our interest rates are contributing" to a favorable balance of payments, he said. Dealing with high interest rates and a strong dollar is "going to be difficult competitively."

Brock mostly discussed the Multifiber Arrangement negotiations -- continuing in Geneva next week -- on renewing the framework in which nations form bilateral textile and apparel agreements. He said that the developed countries will ask the Big Three textile exporters -- Korea, Taiwan and Hong Kong -- to restrain their growth in that industry. But representatives of those countries said they already are suffering, "and I don't blame them" for not wanting to reduce their exports, Brock said.

None of the countries involved in the negotiations has developed a position yet.