I think I know the sound you are talking about. If its what I'm thinking, you are probably getting ready to crap out your rear diff. An easy way to check this to to drain the fluid, but filter it with a shop rag as its going into your drain. When the fluid is all passed your filter look for metal bits. Check the magnet on the drain plug as well.

I've gone through two rear ends on two different Crown Victorias that made a very annoying "wow...wow" that got louder and more intense as it progressed. Apparently, there is an issue with the Ford axles they put in the CV's. I remember there was an issue with the rear ends on the early Xterras, but that should have made itself known by now if it was that problem.