I'm 75% sure that I know who the forerunners are!
343 Giulty Spark said that he was the monitor of installation 04(which clearly states that there are more Halo's). In the missions Silent cartographer and Assualt on the control room Cortana said that The forerunners built Halo for religous reasons, Which means that the covenant prophets had something to do with this;A covenant prophet is a religous leader, very high on the royal pyrimid. Cortana said that the control room was located in a temple or shrine. I believe that the past prophets are the forerunners, and they Had Every Halo built as a gift of some sort to there God(s) so that they can worship there God(s). But when they discovered the flood they sent High ranking Elites(gold elites have Holy symbols on there backs) to contain them and study them, they were ordered to set up A defence system in-case the flood ever got off the ring. That is when 343 GS comes in; they built him to monitor This certain Halo and retrieve the index to Activate the defences if the flood escaped... SUMMARY- the Prophets ancestors are the forerunners...