The "groan of a two hundred year warning bell" can refer to the years of Imperialistic rule over Philippines. The "warning bell" idea can be read as Filipino identity is not something that can be repressed and denied without some type of consequences.
The quotation, in a larger sense, refers to the awakening of post- Colonial life. Imperialism and colonialism sought to increase nations' hold on other nations. The challenge becomes when colonialism ended, these nations had to define themselves, for the in the absence of a controlling figure to give definition, identity had to come from within. This has been a challenge in many nations. Some formerly colonized nations in Africa, Asia, and South America are still in this process of "self- definition." Other nations like India have sought to define themselves in divergent manners on both economic and political grounds. Nations like the Philippines, the subject of Rosca's work, seeks to define itself in the wake of controlling imperalism and colonialism. The "groan" of national identity is something that cannot be denied for too long; it is "the warning bell" whose toll must be heeded.