I've had this issue with my '01 auto transmission for a couple of years now, and am wondering if anybody else has had the same problem. Cruising at highway speeds, whenever the torque converter unlocks, I get a weird grinding noise for a few seconds. It sounds kind of like a two-stroke dirt bike, or like an exhaust heat shield rattle. But it is not a rattling heat shield, and this isn't a dirt bike. It is very loud and somewhat alarming, but it goes away gradually over 2-3 seconds, even sooner at higher RPMs such as when I kick it directly down to 3rd (bypassing unlocked 4th). I have tried swapping transmission fluids twice, and the fluid level is definitely correct. I also replaced the pickup screen, but the old one was fine, and the fluid has always looked good for the vehicle's age and mileage. Shifts are firm and there are zero issues aside from the terrible noise. It has occurred consistently for two years, and occurs only at the moment the torque converter unlocks. My plan is to keep ignoring it, but if anybody else has solved this problem, I'd love to know.