Buzzing sound when hot and under acceleration

[kenbefound]

I have a 2002 Boxster S that once the car warms up I start get this buzzing sound when under acceleration. I can tolerate it until I pass by a fence or any kind of reflective surface and the sound bounces back to me. It really diminishes my experience of the car and I find it embarrassing. I had it looked at and nothing obvious has been found. I suspect it's originating from the exhaust system, but had that checked out and nothing of note found there either. Anyone have a similar issue that they were able to correct?