"Buzzing" exhaust & Tranny question

[subiedo]

Probably a couple common issues but wondering what you think. I've got a distinct buzz almost a nasal passage kinda sound when driving the car under load. Sure sounds like a heat shield vibrating against metal which I recall I had with another older Subaru a few years back. Sound like it's the Cat or something closely related? Second question. The transmission (I'm at 90k & no rebuild) started making a few small "clunks" when I was in reverse a few days ago. Then in Drive, I got a couple more before the problem ceased. It was driven a little aggressively before this occurred and hasn't repeated since. Tranny fluid is pink and at the correct level but has that little burn stink to it. Would it be a good idea to do a new filter and fluid change along with the some Brad Penn LSD additive (& a tranny cooler) & see how it goes? Or does this symptom sound like something more severe getting ready to show itself? Hmmm?