My Z which was previously owned by Dazsw from the above posts had this when I bought it. I had the exact symptoms as noted above, no noticeable power loss but rattle under low load in 2nd gear at low speed. It sounded like an exhaust shield loose which my car didn’t have, or flywheel/clutch related. A year and a half after buying it, I took it for a new Link ECU and the tuner found the passenger side turbo wastegate hose I believe had popped off and was rattling around. Once reconnected, rattle gone! Very random, but this might be your issue. Before the issue was identified, I had the flywheel and clutch changed but issue remained so ruled out that.
