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Will it also thunk if you try to start doing a power brake, in other words if you hold the brake with your left foot and goose the throttle a bit to torque the engine, do you get the same type noise? Sometimes a heat shield for the exhaust gets bent on road debris or some obstruction ans can make noises at certain times when the exhaust hits the shield, as an example. There are ways of physically checking for a worn motor mount. Could be one of those yet, or a trans mount, noises like this are tough to diagnose without being able to look around the car, sometimes if the car is put on a hoist it becomes apparent where the problem is right away to a trained eye.
Motor mounts are typically done in pairs, unless you really don't plan on keeping the car very long. If one is really bad and the other you know is good, you could do just one, in general I'd recommend doing both. Trans mount you can do just that part seperately.
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