With A/C on, 2004 XC90 idles at 800-850 and causes muffler rumble, esspecially in third seat. With A/C off idles at 650-700 and is as quite as a mouse.. Is this something that should be adjustable?

Has anyone experienced rear channel prologic problems? While listening to FM in Prologic mode I get sever static out of the rear channel. When I switch to 3 channel static disappears.