Differential Chattering...

How often do you need to change the fluid in the diff and how hard is it? My problem might not be the diff, but just wondering (my Chilton manual doesnt show how). BTW thanks in advance for everyones help! The symptoms are that when after driving on the highway for about 30 min then when I get to my destination and sit at a light I hear an obnoxious chattering noise... When I park the car it still makes the noise and if I shut off the car and start it up the noise is gone. The sound is definitely coming somewhere near the diff. I checked to see if the mufflers were hitting the tank, but they are not (pretty stiff). Does not sound like squeaking (even though my leaf spring bushings are shot and look dry and PLAN TO REPLACE THEM SOON). I drove to my campuses empty parking lot and drove in tight circles, going left and right, still no noise even when putting back into park. Kinda funny when I saw the campus police watching me. The tranny was rebuilt back in January (payed $900 but the guy knows his stuff and even found that the stupid PO who did work on the car put some parts back in wrong). So if anyone has an idea, tell me if a simple amateur like me can do this (done carb work, ps, some electrical), or can possible tell me of a better manual to buy that is more detailed than the Chilton (cant be on a disk because I have a Mac). THANKS!

I almost forgot, could it be the yoke? I have not checked the gasket or what condition it is in, an the mechanic didn't say anything. Work gasket for the yoke causing vibration thus the chattering???

My mistake. It is an auto tranny.I have isolated the noise (more of a chattering) to under the car, from tranny to the diff, but from what i remember its near the diff. I will test drive tomorrow (get on the highway) then start diagnosing what and where the problem is. I hear it mainly at low speeds and occurs when I shift into park, then when I cut the engine and start it back up the noise is gone... Then after the engine has been shut off, I drove only on the streets to the parking lot, did my test, and no noise. I will report back later today since I am really tired and don't want to drive going 60mph with no sleep.

Weird... I drove 50 miles today (20 were highway) and no noise... My only concern was that the crankshaft pulley is wobbling (maybe 1/8" ). It wobbled like this after driving from the highway, but after driving 30 miles in the city it wobbles a little more than 1/16". The alternator, PS, and water pump don't wobble. tried to get a video recording of it, but hands are not steady. I Checked to see if the starter bolts were loose, they weren't. I will just keep an eye on her and watch for anything else.