This is rather confusing, in that you say it goes with road speed (not engine speed) but also say that it only happens when you take your foot of the accelerator. Have you tried driving slowly and then depressing the clutch so that the engine is idling? Can you hear the knocking sound then? This would at least eliminate the engine.

You may try going to an area (such as a supermarket car park) when closed and driving the car at approximately 10-15mph. Turn the steering fully to the right lock and then the left lock, and see if you can hear a knocking sound. If so, it will be the driveshafts. Other than that, jack up both front wheels and rotate them, and also pull them from side to side to see if there is any knocking sound. This may indicate worn steering ball joints.