This has been discussed in many other threads. It is due to the rear differential settling during hard shifting .. there supposedly is a bit of free space in between the rear diff and the chassis(?) - there is a fix out there that you can get if it bothers you though. Some sort of diff rings or something. Search for rear diff or clunking
it doesnt really take away the full thump from me, but it does lessen the sound in the rear diff. the old thump that sounded metal hitting metal, now sounds less like a rubber hitting rubber sound haha, i dont know how to explain it
yea my car does the same clunk when you are really mashin the gas and letting the clutch fly. I found that allowing the clutch to slip alittle while you get back into the gas takes it all away and still gives the car some serious get up and go. try it and see if that works for ya.
Glad to see you are breaking it in nice and aggressively, thats how i broke mine in, and i have no problems w/ burning oil