M35 2006 Manumatic shift knock???

[Corvettelvr73]

Ive had my M35 for a couple months now and i love it. I have recently tried out the manumatic shifting and ive noticed that sometimes under load it seems that when shifting to the next gear it shifts hard and makes a knocking sound comming from the rear. Almost like the sound that happens when hitting a bump or pothole in the road comming through the aluminum suspension. The sound deffinitely is comming from around the trunk or under the rear seats because a passenger was back there and noticed that he felt it knock hard under the seat. I was wondering if anyone has experienced this? It doesnt happen all the time but it happens. What could it be ??