Friend of mine has a 2004 outback limited with an automatic transmission. It doesn't have VDC badges on the tailgate, shifter, dash, or grill. No AT TEMP light flashing on the dash. The ATF is ever so slightly dark, but very red, clear, and doesn't smell burned.

 

When he's cruising or accelerating at any speed there's a rhythmic thumping from the back of the car, and you can almost feel it in the seat. It sounds pretty bad. It does not happen on decel.

 

It happens always whether he's cornering OR GOING IN A STRAIGHT LINE. The issue goes away completely with the FWD fuse. It definitely isn't a torque bind issue, there's no binding or whatnot doing circles in a parking lot. Been through that with an ex-car so I know exactly how it feels.

 

I crawled under the car, rear axles are tight, no rips in the boots. All the driveshaft u-joints, center bearing, and CV are tight with no play. Only concerning thing I saw was 1/8th to 1/4 rotation play in the driveshaft with all 4 wheels on the ground. I don't think there should be that much play in the diff.

 

I'm thinking the rear diff is buggered and suggested draining it and checking it out. That would be the best part to fail, they're cheap and painless to swap. 

 

What's you guy's opinions on what could be going on?

Edited December 28, 201411 yr by 987687

I don't know what the break down is between differential failures and driveshaft failures but I'd wager that for every differential failure there is probably 50 driveshaft failures. 

Check the rear driveshaft front U-joint!

 

Most frequent failure.

I did, it wasn't loose. I didn't remove the driveshaft from the diff, but I gave it a good hard shake from under the car, and it was tight. Should probably unbolt it and check, though. It's a really easy thing to do. That and drain the diff... Probably needs a flush anyway.