Suzuki Sidekick/ Geo Tracker not sure which

Anyway, I just did a searched the net for this exact problem, lucky I found it here.
I have a knocking (like 2 really quick knocks) when I accelerate.. It has good power and acceration throughout, but who knows, could be better if it is timing.
I did a coolant drain recently, and I started overheating today, and found it needed more coolant. I also noticed that some of the old green coollant came to the surface. I thought I had it all pressured out but I guess not. I hope I have scored a cyllinder, not even sure if that makes sense, as Im not a mechanic.
Mixed coolants, is that bad, detrimental or a non issue?