Change the tranny fluid or check the levels? Also how fast are you going when downshift from second to first? I only ask because sometimes when I approach a stop sign or light at slow speeds (5ish mph) and downshift from second to first there is a lag with rev match.
The 7at is a clunking tranny to begin with. I have had the thud noise happen once when I was making a right turn into a driveway. I thought I got hit or ran over something. Does your clunk happen all the time? Does the tranny slip into gears? And are you sure it's the tranny and not the rear diff bushing or rear axle? Do a search on the rear axle click and rear diff bushing. Maybe one of these is the culprit.
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2016 7AT GM Base: Fast Intentions CBE 18" Res, Z1 Rear Diff Cover, Quaife LSD, Nismo S-Sport Suspension, Nismo Body Brace Kit, White Line Front Sway Bar, SPL Front End Links, Energy Rear Bushing, Z1 Post MAF Silicone Tubes w/ K&N Drop-in Filters
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