Revisiting this issue...

Could the problem be caused by diff fluid that is a little too thin? I'm running 75W-90 Amsoil in the diff. I know most people recommend 75W-110 instead. The issue seems to be worse when the car is really hot vs when it's cold.

I also get no perceptible vibration on acceleration.

 

Not so much a thumping sound, more like a vibration. Really, it feels like a locked diff or very tight clutch diff with the inside and outside tires alternately gripping/slipping.
As a rule I am never in neutral while cornering (no one should be!), so I can't say off-hand, will try it. Never ever launch the car, but can get wheel hop in cool/cold and/or wet on RS-3 rear tires. The two issues could be exacerbating one another. Anyway, I've noticed it for literally years/10s of thousands of miles/several track days, and it hasn't blowed up yet (fingers crossed)

 

What tire pressure is to the tires is what diff oil is to the LSD (diff)

 

^^ running amsoin 75-90 is just fine. your diff might be going.

 

I'll be changing the oil in the next few days with 75W-110 Amsoil. We'll see if that makes any difference.

 

i currently run the 110. it works great for my application and i havent had any issues with it. 1 full season of autocross launches and so far so good. :cross fingers: :knockonwood:

 

I have a video of the phenomenon here. A good example of the weird diff shake/noise is at 3:59.

https://vimeo.com/80668592