I've felt this on a heavy 74 out of Singapore before. Was worried at the time (I was about 14). Now that I fly the 767, we get it a fair amount of the time, especially when heavy. As you rotate the nose, it gear extends on the oleo with a thump and you often get vibration. It is like hitting something as soon as you rotate in sound and feel. 767 nose wheels are known to go out of shape and vibrate - I imagine 747 wheels are the same. With a heavy aircraft, it will be travelling faster at rotate and the oleo will have more travel due to higher compression when loaded on the ground.
If the CSD reported it to the Captain I wouldn't worry. If it was anything like the pre stall buffet mentioned above (extremely unlikely) it will be picked up almost straight away and dealt with by BA's safety management systems - which are excellent.