Here I go again with another question for the experts, rear end grinding sound?

 It starts only after being driven for more than roughly 4-5 miles or more. It’s not continuous. Personally I think it's more noticeable during turns (but my wife says it does it all the time)

 I’d say its feels like one certain spot during tire rotation. So I think that would rule out bearings right?

I recently changed rear brakes and emergency brakes. But the sound was there long before that.

I thought about changing the fluid in the differential because of posts suggesting doing so and I figured it couldn’t hurt.

Any thoughts or suggestions are appreciated as my local mechanic’s going rate is 100 an hour. 

Check the play in the side yokes on the diff. Jack up the rear and have someone hold the tire at 12 and 6 oclock and check how much play they got at the diff. If more than a 1/8" or so they could be worn down on the inside. (Not sure what the correct amount of play is but it should be under an 1/8) They could wear to the point that the "C" clip falls off and allows them to move out too much. Also it could let the outer bearing seal shield  rub against the diff from pressure during cornering.  '78 was one of the years with side yokes made of a very poor quality metal. Mine were worn pretty bad with only 47,000 miles.
If the noise is the chattering starting off in a turn mostly then new oil and a bottle of posi additive usually helps a lot.
http://i159.photobucket.com/albums/t132/JeffsVette/DSC03587-1.jpg
Jeffm2010-11-12 18:32:43

Oh boy trying to figure out a noise in the car without hearing it..well if you think it does it more on turns but the wife says it's there all the time I would guess a wheel bearing in the back is dry..wheel bearings make some noise going straight then change or get worse when you turn..see if it is worse turning right or left..it should make a difference..unless both are bad..inner that is..I know when mine smoked I had very little warning..like 1 mile..I think that I heard a few noises here and there but it's an old car..so I didn't check into it..might try and get some grease in the wheel bearing and see if it changes..my outer bearing was actually fine but the inner exploded..so I guess the need is to grease the inner which really is the only one you can..put the rear in the air and check end play one the wheel..if you can feel it or really hear it chances are the bearings are worn out..or a dial indicator will tell all.

My vote is for da spindle(wheel) bearing(s). A bad spot on a bearing race will make a noise every rotation like that...like a "whomp-whomp-whomp"....

i know that its hard to diagnose car problems through a forum but it will help me with things to check tomorrow when i lift her rear up and check her out(the vette!)

[QUOTE=Jeffm]Mine were worn pretty bad with only 47,000 miles.
http://i159.photobucket.com/albums/t132/JeffsVette/DSC03587-1.jpg
[/QUOTE]

DAaaaang, Jeff!! You're lucky the snap-rings didn't fall off of those puppies! Looks like the one on the right was just about to go MIA!

[QUOTE=Adams' Apple]

DAaaaang, Jeff!! You're lucky the snap-rings didn't fall off of those puppies! Looks like the one on the right was just about to go MIA!
[/QUOTE]

Yes sir Joel, the oil was like a metallic pudding from them ends wearing away. yum, yum.
You guys are probably right if it's more of a steady sound. I never did any bearing or trailing arm stuff but hear it's a real pita which is why it's never serviced on a lot of these cars.
What I know is only what I have managed to work my way through and that's with a heck of a lot of help from this forum of course.
Jeffm2010-11-13 07:03:48

The rear wheel bearings aren't really that hard..they just take a short handled heavy hammer and either a spindle nut tool or the press tool..a set up tool and a install tool..all sold at mid America..the grease tool is something I highly recommend also..if you managed to change the rear shoes on the car with the spindles in then the bearings would be only a little more work..patience is the word for this job..I would change the diff fluid anyhow since it probably wasn't done in some time..don't forget the additive also..
Rich