Our 2016 s60 makes an inconsistent (no rythym cycle), intermittent faint rub sound whether just starting out, running a long time, sitting still, driving fast, slow, uphill, down hill, full of fuel, half empty, etc, turning, going straight. No dash warnings. it's been doing this for months. Certified volvo mechanic heard it, ordered and installed new gas cap. No change.
any ideas what is causing tge sound?
help!