Front Axel or Transmission Thumping

tzarigor

I own a 2001 Legacy Wagon L with 123000 miles on it. It developed a thumping/grinding sound in the front when turning sharply at slow speeds in either direction going forward or back (like when pulling into or out of parking slots).
I took it my mechanic and they were convinced that it was the front axels which they replaced with rebuilt ones. The problem briefly went away but then returned. They replaced it with two more rebuilt axels without fixing the symptoms. Now they think that it is the "front differential" and suggest popping in a complete rebuilt transmission unit for $2300 .. ouch.
I have no problems with shifting or the clutch at any speed. So, how could it be the transmission? Or, what could be making the thumping sound when turning slowly? Again, no problem with turns when done faster than 5 mph.

dcm61

Center Diff (aka Viscous Coupling)?

tzarigor

How would the center differential create the thumping noise only when turning sharply at slow speeds? I would suspect I would hear problems in other situations.

xwesx

Most likely, the reason you don't hear it at higher speeds is because you cannot turn as sharply at those speeds. At 5 mph (or less), you can turn the car at full lock, which puts a lot more stress on the differential than high-speed turns.

I suspect that the front differential is the most likely culprit in this case. The half-shafts can make noise when the CV joints go out (most likely because the boots tore at some point and allows the joints to go dry), but the sound is more likely a grinding noise and tends to only happen when turning (slowly) in one direction. When the joint gets really bad, it will start to make noise all the time (again, it is a grinding noise).

Why would you need a whole new transmission if the problem is the front differential? The simplest explanation is because the entire unit (transmission, center differential, and front differential) shares a housing, called the "transaxle," and it's tough to work on one part of it without impacting the rest. For an automatic transmission, the front differential and transmission are in separate, sealed housings, and use different fluids (gear oil vs. ATF). Again, though, I'm not sure you can work on one without cracking open the other.

The good news is that you have time to plan for this if you decide you want to pursue it. Although, given your current shop's track record, you may want to pursue a second opinion....