The "new" Aerostar I just purchased (93 AWD shorty, 132K) came with free rear end noise. Assuming in advance I'd have to repair or replace the differential, I recalled in the past when I posted questions about this in regards to my other van (turned out to be front wheel bearing) that loose pinion nuts or other stuff could make this noise. Here's what I know:

The noise goes up with increased speed, but seems to quiet down at higher speeds (50+). No visible leaks around axle. No noise when decellerating. No vibration. U-joints seem fine (no play). Any suggestions as to how to troubleshoot it? I'm assuming that, if it inside the diff, the best course of action would be to replace the whole axle with a wrecking yard unit which I'm pretty sure I can get for less than $200 around here. I feel pretty safe as these are generally strong units, or seem to be. It's a standard 3:73:1 axle ratio (not traction lok).

Thanks,
Mark Biederbeck

 

I have the same problem with my 87 aerostar. I did flush the rearend oil and replaced it with a synthetic rearend oil and it helped to some degree and should help keep the rearend going longer. I do feel that this may be a ring and pinion problem and the whine is a symptom of that type problem. I've been told that MANY Fords, even new ones, have this problem of a whine in the rearend. I did remove the differential cover plate when I changed the rear end oil and checked all the gears and they had a little wear but not bad at all. You might want to try the synthetic oil change. If your differential cover plate is the plastic one, Ford recomends replacing it with a new cover plate if you ever have to remove it. There seems to be a problem at times with resealing the old plastic cover plate. Ford has even told me that they have replaced whole rearends with this whining problem and the problem still exist in the new or rebuilt rear end.
I've got a feeling that this is an inherent problem from Ford. I don't think this is a real serious issue that calls for an entire rearend replacement. I would suggest doing a rebuild or partial rebuild when you decide to deal with the rearend issue. A junk yard rearend might whine worse than yours from my understanding. Hope this helps.

 

Another thought. If you havn't checked, pull the back wheels and break drum and make sure you don't have a leaking grease seal.