CRX Auto Tranny issues

Today I was parking with my window down. As I moved the transmission from D into P I heard a clicking noise. I couldn't tell where it was coming from other than it was likely under the hood. It has felt like the transmission is slipping a litte. I haven't had any problems with the way it runs and it hasn't affected my ability to drive it. I am really worried tho, any advice or thoughts on what might be wrong?