Help with diagnosis

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Ok, my car recently developed a thumping sound that varies with speed (tire rotation) and not rpm. It started as a faint sound like tire pickup on the left rear. As I drove home it became more like a nail in the tire, then a lag bolt in the tire, now getting louder more mechanical sounding. I changed the wheels, inspected the brakes and rotors, inspected the CV joint boots and all looks a-ok. The wheel bearings are 6 months old. Preliminary diagnosis from my trusted Porsche mechanic is that the trans is going south.

What say you??

This is a 98 2.5L Boxster 5spd with 97K miles and 50+ track days. Trans fluid is changed annually with no obvious signs of serious problems. I have noticed that she has been stiffer shifting lately but attributed that to cables and bushings. I generally shift gently and have never had a synchro failure. I adjust my driving style so I rarely generate wheel spin on the track.