Does this never occur below a certain speed, typically around 40 mph? If so, once you reach that speed, hold the gas pedal steady and quickly tap the brake pedal and release it. You should feel engine speed pick up about 200 rpm, then go right back down one or two seconds later. If you feel that clunk, that is most likely a shudder from the torque converter locking up for better fuel mileage. The clue is you can repeat that test over and over while cruising above that minimum speed when it will engage. While that might be an indication of a problem, on some transmissions it is a result of using the wrong transmission fluid. The shudder is caused by the clutch grabbing and slipping repeatedly until full fluid pressure is achieved. By "accelerating through it", (which is a dandy observation to share), the locking of the clutch may be occurring after the load on the transmission has been reduced so the lockup clutch has less torque on it and is less likely to shudder.
If you can change the feel of the clunk by how you accelerate, also look at the engine and transmission mounts to see if the rubber isolators are torn or deteriorated. You may feel something unusual when you shift into reverse too.
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Sunday, November 11th, 2012 AT 4:21 AM