Having used the car now for a couple of days I can say that the noise is no more.I took the car to main BMW dealer and the mechanic sat in the car with me.....Of course the noise wasnt occuring..typical.Anyway I played a recording of it to him and although they had replaced the fuel line under warranty, he said that the noise is related to the HPFP.In many cases replacing the fuel line only does solve the problem but if not, then the pump needs to be replaced.Although the pump was working with no fault code it was creating an uneven flow of fuel causing vibration of the fuel the pipe.Replacing the pipe often solved the problem because sturdier clips are used to make the pipe more rigid and thus eliminating the noise.I guess this is the stock remidy that BMW adopted as it was not necessary in all cases to replace the pump . So for anyone coming across this post with humming/croaking noises,be assured it is the HPFP causing the problem and if you are lucky, replacement of the fuel pipe might stop the noise but if not then the pump will have to be replaced.I am assuming they have upgraded the pumps now.The BM mechanic was not sure what caused the actual problem in the pump but added that it is the first sign of the pump failing and could be due to cheap supermarket fuel corroding the internals. The 'noise' is a known issue at BMW and incidently in North America there was a blanket recall to replace the pumps on certain year cars. The BMW dealer really looked after me on this and although there was not a fault code generated by the HPFP, they replaced it under the 12 month used car warranty. I 'rest my case' and now can start enjoying the car...Thanks everyone for you input on this and I hope my re telling my experience will help someone else with 'frogs' in the engine.