The engine noise in the GMC's is purely caused by the Bose / Onstar interfaces. It's an issue with the modules themselves. I've used the GMOS04 from Metra, the Soundgate module and as of yesterday an Axxess from Best Buy.
The Metra, had the engine noise unless the amp gain of the module was about 10% or less for me. The metra also had a firmware bug where the door chime would chime non stop for 10 minutes (even if the doors were closed) if you closed the door at the exact same time you put the key in the ignition (exactly why I dumped the metra module).
The soundgate module fixed the door chime bug because it had a newer firmware, but it had a bug where it would loose the amp gain setting randomly (about 1 day every 2 weeks of driving) and even with the gain wheel on the module set at about 15% it would kick in about 50% for no reason (my guess is cold temperature or something). The only way to get the module to go back to the original setting was to disconnect it completely (fuse pull) for a few minutes and then hook it back up. That was fun because I'd lose all my presets and some Z2 configs.
My soundgate module after 9 months of owning it started draining my battery randomly as well. I had a 2amp current draw from that circuit with the truck off. I got tired of going out to a dead battery every few days in my truck so I just yesterday dumped the Soundgate module.
I bought the Axxess module from Best Buy and hopefully that'll resolve my isuses.
I will note, ALL of these modules are made by the same company and they're just rebranded for whoever you buy them from. The Metra and soundgate were identical boards and wiring harnesses and just different firmwares flashed on them (likely an age of shop inventory more than anything for what firmware you get). The Axxess module I picked up yesterday is still a GMOS04 module, but they're using a 2006 dated board (the previous Metra and Soundgate were 2004 stamped boards) and the newer board has a new 14 pin connector on the one side instead of a 10 pin (the other 24pin is still the same across all 3 modules). I'm hoping the newer hardware solves the random amp gain issue as well as the battery drain.
One other nice tip for those of you with GM's is the new module with the 14 pin connector brings you VSS to the module instead of having to chase it down at the instrument cluster. This would have saved me time on my original Z1/2 install 18 months ago when I had to hunt around for my VSS wire in the instrument panel.
Anyway, hope this helps some people, I'd like to know my $300 in multiple Bose/Onstar modules is going to some use other than fixing annoying bugs with crap firmware.
Oh, BTW, the new module (2006 board with 14 pin connector) also has a connector on the module that looks like it's for flashing firmware updates. Now I just need to figure out how to flash it to newer firmware when issues are fixed by the OEM's instead of having to buy a new $100 module every 6-9 months.
If anyone has questions about the firmware versions, hardware revisions or other info about these modules let me know. But I'm quite confident the GM engine whine issues people are referring to are 100% attributed to the interface modules themselves and not a ground loop in your head unit wiring.
Ralph