2007 Toyota Avalon, 142,000 miles, we feel a clunk on deceleration when the transmission abruptly downshifts around 20 mph. Not always, but particularly when we have to slow more quickly than normal for traffic. My suspicion is an issue with the torque converter lockup, but I don’t know what to do about it. Is the tranny toast?
Those exact symptoms can be caused by a low level of fluid in the transmission.
Have you checked the trans fluid?
What about an engine/transmission mount?
Back in about late 2007/early 2008 I went into the parts departement at our local Chevy/Buick/Toyota dealer. I noted they had 3 near new Toyotas on racks up near the service desk. The transmissions were out of them all and on the bench. A sign of things to come?
In passing I mentioned it to the service writer and he said yeah, they’re having some issues. Being in a rush I did not stop to find out what issues. Maybe yours has become a problem child for the same issues?