2004 Kia Sedona

I have a 2004 Kia Sedona and it has been sitting now for about 4 months because I had a knock in the engine and a loss of power. A local church group has a car club that fixes broken vehicles for people and I took it to them. They removed the shroud that covers the engine and a piece of carbon flew out from on top of the piston and the rattle stopped! I was excited the van seemed to be running good for about 2 weeks and then it happened again. I hate to call on that group again and was wondering how can I fix this myself, and what is causing the carbon build up?
August 24, 2011.

Did the piece of carbon come through the spark plug hole or was it sitting under the shroud, (valve) cover?
Are you sure it was carbon?
I ask because carbon is soft in most situation unless it is built up on top of a piston.
It would be un-likely for it to make a rattling, "metallic" type sound.
If it did come from under the valve cover, it is dis-concerning that that much sludge or a piece of carbon ended up their.
If the carbon came from a spark plug hole, that is build up on top of the RPMs or not using a good fuel system cleaner.
If it came from under the valve cover, it is either a piece of metal or sludge from not changing oil and not running fuel system cleaner.
Or if you have not kept your ant-freeze serviced, you could have a blown head gasket causing damage.
Generally it sounds like motor damage, lack of maintenance or something there-in.

Does the vehicle have any performance problems?
How long did it rattle before you had it looked at?
If you ca get another piece of it, I would look at it under a good light and magnifying glass and maybe even try to clean it off to see if it is not carbon covered metal.
Let me know how it goes and I will watch for your reply.