Only at idle. Initially thought fan, but was coming from rear. On a lift, sounded like tranny- mechanic pulled off rectangular rubber cover showing spinning clutches(?). Took to tranny shop & they said not tranny or “torque tube”. Also could not recommend where to go next - Northstar related or Corvette?? Caddy or Chevy dlr?? Just moved to Durham, NC a year ago - have seen exactly 1 other XLR! Any ideas for step 3 would be greatly appreciated!! Thanx!