My 2007 CTS Sport with the 5 speed automatic is making a odd buzzing/whining sound at idle in manual mode. It only happens when the transmission is in sport with manual mode engaged. The noise does not happen in drive or in sport mode without manual mode engaged.It happens with 1st gear selected while idling at a stop light or stop sign etc. Anyone else have this issue before? The car has about 70K miles by the way.