The "flip-flop" noise you described, if it occurs at the end of the stroke (when wiper direction reverses), sounds like what I used to get once the blades became contaminated with whatever fluid they use at the Dealer. It is the sound of the blade flipping over because it is sticking to residue on the glass.

Here is a quick test. Wash the windshield, then place a dry rag under the entire length of each blade. This should reduce friction and allow the blades to glide more easily. If the noise still persists, it is probably in the mechanism. In that case, I would be concerned about somthing having become loose that may soon get worse.