An upshift too quickly can create a knock and I wouldn't be surprised if some driving conditions bring that out; I have noticed the same issue with my 2014 3.6L and six speed where it upshifts too quickly (done for economy/emissions reasons) and on a grade some light spark knock is obvious. But if you notice any heavy knocking or other unusual noises or operational behavior keep looking because the ATS isn't a scarce vehicle and plenty of others are available.
Outside of a torque converter issue caused by GM programming along with some questionable emissions/economy oriented programming in some models the 8 speed has some pretty nice features. I have a 8L90 in my 2016 Z06 and the 8L90 is very similar to the 8L45 used in the ATS but is built to withstand much more torque. I mostly use mine in manual mode to avoid GM's active fuel management system but driven hard in full auto it has very nicely timed shifts, both up and down, and with the car in sport or track mode on a twisting road the programming shines. Driven around town in auto, the shift algorithm is aimed too much at smoothness and can be lethargic at times.
From complaints I have read the 8 speed programming is far worse for some models with indecision and slowness to choose a gear but I haven't experienced those issues. For the first start of the day I manually shift from first to second while the car is still stationary to avoid a rough shift on the initial 1 to 2 upshift which is due to a slow filling hydraulic actuator used during that shift but it isn't a big issue since you only need to do this preventative behavior on the first start of the day. The only time I have seen the transmission get confused about gear selection is when you slow almost to a stop and then get back into the throttle; the transmission skips second when coasting to a stop and if you get back into the throttle just as it is slowing to a stop and starting the 3 to 1 downshift it will get momentarily confused about whether to stay in third, shift two ranges to first, or drop one range to second. Otherwise I haven't discovered any 8 speed shift anomalies; the last chance I had to drive the Z06 before winter I was on an interstate merge ramp when the minivan in front of me decided to slow to 35 and then pull over. I rolled heavily into the throttle while still keeping power controlled so that the summer tires wouldn't break loose in 45 degree weather and easily got ahead of a large pack of traffic while the right lane was clear; the transmission shifted quickly into lower gears and then upshifted quickly at near redline. When the 8L series transmissions are pushed hard they do very well and I prefer it to the 6 speed in my 2014. But the torque converter issues (shudder due to GM converter clutch modulation strategy) made it an easy decision to replace my 2006 GMC 2500HD with another heavy duty diesel which comes with a tried and true Allison 6 speed auto. I hope GM learned their lessons and hopefully the new 10 speed co-developed with Ford doesn't have as many annoying behaviors. There is at least one class action in progress against GM over the 8L45/8L90 transmission and the third new fluid formulation since it was introduced is the latest attempt to mask the converter clutch problem.
Outside of a torque converter issue caused by GM programming along with some questionable emissions/economy oriented programming in some models the 8 speed has some pretty nice features. I have a 8L90 in my 2016 Z06 and the 8L90 is very similar to the 8L45 used in the ATS but is built to withstand much more torque. I mostly use mine in manual mode to avoid GM's active fuel management system but driven hard in full auto it has very nicely timed shifts, both up and down, and with the car in sport or track mode on a twisting road the programming shines. Driven around town in auto, the shift algorithm is aimed too much at smoothness and can be lethargic at times.
From complaints I have read the 8 speed programming is far worse for some models with indecision and slowness to choose a gear but I haven't experienced those issues. For the first start of the day I manually shift from first to second while the car is still stationary to avoid a rough shift on the initial 1 to 2 upshift which is due to a slow filling hydraulic actuator used during that shift but it isn't a big issue since you only need to do this preventative behavior on the first start of the day. The only time I have seen the transmission get confused about gear selection is when you slow almost to a stop and then get back into the throttle; the transmission skips second when coasting to a stop and if you get back into the throttle just as it is slowing to a stop and starting the 3 to 1 downshift it will get momentarily confused about whether to stay in third, shift two ranges to first, or drop one range to second. Otherwise I haven't discovered any 8 speed shift anomalies; the last chance I had to drive the Z06 before winter I was on an interstate merge ramp when the minivan in front of me decided to slow to 35 and then pull over. I rolled heavily into the throttle while still keeping power controlled so that the summer tires wouldn't break loose in 45 degree weather and easily got ahead of a large pack of traffic while the right lane was clear; the transmission shifted quickly into lower gears and then upshifted quickly at near redline. When the 8L series transmissions are pushed hard they do very well and I prefer it to the 6 speed in my 2014. But the torque converter issues (shudder due to GM converter clutch modulation strategy) made it an easy decision to replace my 2006 GMC 2500HD with another heavy duty diesel which comes with a tried and true Allison 6 speed auto. I hope GM learned their lessons and hopefully the new 10 speed co-developed with Ford doesn't have as many annoying behaviors. There is at least one class action in progress against GM over the 8L45/8L90 transmission and the third new fluid formulation since it was introduced is the latest attempt to mask the converter clutch problem.
