TAIPEI, Jan 11 (Reuters) - Taiwan’s central bank has sent inspectors to domestic banks to investigate whether exporters are speculating in foreign currency as the Taiwan dollar soars against the U.S. dollar, three sources with direct knowledge of the matter told Reuters. Taiwan’s currency, which is up 1.4% against the U.S. dollar this year, has vexed the government, wary of being labelled a manipulator by the United States, the Chinese-claimed island’s most important backer. Last month, the U.S. Treasury added Taiwan to a “monitoring list” of countries whose currency practices had caused concern, the first time the island has appeared on the list since 2017. The sources said the central bank, concerned about exporters speculating in foreign exchange through overseas borrowing, had recently dispatched staff to banks to look at large-scale remittances from their customers. The inspectors are looking at any abnormal increases in foreign exchange volumes to discover whether manufacturers are taking the opportunity to speculate in foreign exchange, the sources said. “The central bank has started to investigate large remittances by customers,” one of the sources said, speaking on condition of anonymity as they were not authorised to speak to the media.