That's a very good point BnB that it does not happen on Neutral is a good test. However, the engine is doing a great deal more work when it is under the load of the transaxle moving your car down the road than it is just sitting there in neutral. Unfortunaly if it is the cats (and it might not be since there are a bazillion freak things that go on with this car) but if it is, there is no real good test for it.. at first. One ofthe tell tale signs is that the noise will eventually be followed by a loss of power. If you start expiriencing a building loss of power, it's nearly certain. My cat went caput during a 425 mile drive from FL to NC. On the way home it was undrivable untill I removed the main cat and pipe that's right after the y-pipe and there was literally a big chunck of honeycomb cat lodged in there. I removed it by crushing it and was able to get the car home.
Here is a test that may work for you. The noise is the air trying to find some place to escape because of the back pressure of the cats. I found that when I had them failing and my car was around 30% of it's original power, that loosening the bolts on the exhaust flange just after the y-pipe and cracking it open about 1/4-1/2" returned the car to about 75% power. While the main problem was in the pre-cats of the exhaust manifold, the back pressure was still the sum of that pressure and the pressure of the main cat (mine was pretty clogged) as well as the muffler and other pipes. What this means to you is that if you crack it open, they whooshing sound very well should go away b/c the escape pressure would be relieved (unless the cats are really really bad and it doesn't sound like yours are). The tricky thing is that your exhaust tone would of course be very loud at this point so you would have to differentiate but I think it's doable. I hope this helps, and I hope that it isn't the cats but someting else that is simple. The cats problem is not fun.
