Help: RFE of I-485

Hi,

My I-485 is pending for over 3 years because of the name check with FBI. Just today I received an email from USCIS saying they mailed out a notice to request more evidence. I have not received the notice yet, and don't know what they need. Is this a good sign or bad one? I have mixed feelings: happy because finally somebody is working on my case while concerned about the evidence they need. Anyone had such experiences before, could you give me some insights/guess? Thanks a lot for your time.