Send me a 986 (in a crate, on the back of a train or ship) and I'll work on it for you. Better send an extra one that I can keep and resell (not a car I would keep for myself, ugh.)

BTW, this thread is about a Boxter, isn't it? Did you grind/sand absolutely ALL the paint from the spot where you're grounding? So the ground terminals are touching a nice patch of bare metal? I'm not second-guessing you here. This is something that's obvious to me, and may very well be to you, but isn't to everyone. No bolt will provide an optimal ground despite what some people assume.

Anyway, there is obviously some difference in your setup and Clownburner's. The only thing it is safe to assume is that the empeg, barring any type of defect in your specific unit, is a known-stable platform. In some vehicles no matter how you ground the sources you're going to get noise for other reasons.

As Hugo pointed out, bad alternator connection as an example. Poor quality interconnects. Most sex/sex straight connectors are generally poor and shouldn't be used to extend interconnect. Damaged interconnects. Interconnects running parallel with other wires in the vehicle (note that the entire chasis is carrying voltage as well and you can't get around running wire near the chasis

This is the longest running problem in car audio. And still quite possibly the most elusive. But it always (always) pays to start with the basics. And the basics are enough to get the majority of people out of trouble.

Oh. And it's amazing how many installers (people working at install shops) don't know even the basics.

Bruno