I have a 2001 Ford Windstar with 102,000 miles. Sometime over the past few months it started to get a periodic vibration. It lasts about 1 second (maybe a bit less), and feels like you just rolled over a series of 2" pipes. It's most noticeable on smooth highway driving, but I'm pretty sure I've felt it on surface streets also. They are just so bumpy around here that often it is hard to tell. Eventually I realized it only happens when the A/C is on, and is more frequent when it is hotter outside. Generally it happens every 30 seconds to 3 minutes, and it usually happens within a couple of seconds of first turning on the A/C. For this reason I'm assuming it happens whenever the compressor starts up.
The Ford dealer is guessing that it is the torque converter, but that's only a guess. It will take a $2200 repair to find out, so for now I don't plan on doing anything unless the torque converter fails. I asked what the tie-in with the A/C is, and the best they could do was speculate that the added engine strain somehow triggered it. It sounds pretty fishy to me, especially since the transmission is rock solid (by both my experience and their testing). A transmission flush didn't help at all.
Any other ideas as to what the problem might be?