More than likely your transmission is not aligned correctly under your car.
The top of the c-beam should be 45mm from the bottom of the floorpan and the c-beam should be 28mm from the drivers side.
The buzz you hear at WOT is the transmission riding up under the car and some part, maybe the shifter, maybe the transmission, is touching somewhere and transferring the buzz into the cabin.
I just learned this while talking with Marc Haibec last week. I had asked him a few questions about shifters & boots when I was replacing my transmission. I asked him how important it was to align the transmission and that's when he told me that if it wasn't exactly where it should be that under WOT the car would buzz just like you describe.
If you have stock exhaust there's just enough room to get at the c-beam bolts to loosen them up. Just loosen them and let the transmission hang down on the bolts. Cut two pieces of wooden dowel, one to 45mm the other to 28mm. Use them to measure the top and side of the c-beam to the body of the car. Adjust the transmission by lifting or lowering it on a jack and or prying it from one side to the other.
Then re-tighten the c-beam bolts.
There is no easy way to get a wrench on the nuts on the top of the c-beam. Be prepared to get creative and be prepared to need band-aids and BenGay afterward.
Bring it on by the house, I'll fix it up for you.
TomC