Experts: I'm going insane, I can't figure out this clunk/thud.

[N2MYE30]

I too am having a very similar sound OP under the same conditions but I thought my problem was bad u-joints. This sound happened after my manual swap so I did not know the health of my driveshaft. I did preload the new CSB during my initial install but I may have to recheck.

In for more ideas. :popcorn:

[Sail Boat]

Give various parts of your exhaust system a wedgie, especially anywhere that it's close to anything else. Torque changes can cause things to twist just enough to bang around. If you have a Go Pro, set it up to see how much your engine mounts are flexing when you accelerate and decelerate. Transmission mounts could cause a similar situation. You have a 5 speed or an automatic?

Looks like you were right! After installing a new exhaust, the thud has disappeared.