Once you switch to synthetic oil it is HIGHLY RECOMMENDED that you DON'T switch back. Try a thicker oil. Also how many miles on it. Synthetic is smaller at a molecular level, so it can find all the little holes and bad spots the natural oil can't. One thing I have heard is these two tricks, never done either myself, but I have heard about 20 people mention it.
1. Try draining a quart of oil and replacing it with tranny fluid, this will help clean internally, since tranny fluid is like 70% detergeant.
2. Also I guess this CD2 is good at it also.

Try those to get all the galleys cleaned out to the top end. Lifters and such can start to knock easy when there is a lack of oil. Maybe the clean small synthetic broke up a clot and now its lodged in a galley way.
Don't know for sure, but thought I would give ya something to try. Just opinions, all except the changing back to reg. oil that is.

CJ