I am also having this issue
I also have a Venue Sub12 with these symptoms. Purchased in 2008, started making these sounds in 2014. To me, it sounds like a power discharge through the woofer after the unit has entered standby mode. The clicking at the very end sounds like the relay that controls the power amp. Right now I'm trying to research more details to make a decision between repair and replace. When the unit is on, it still sounds great. I don't think it is a ground loop issue or a bad RCA cable, and all my components including the receiver haven't changed since the initial setup in 2008.