2004 E46 325ci sport - constant humming noise

Hey Guys,
This is a first post for me on here, its a tad long and i apaolgise. For the past 2 years, i have had an annoying electrical type humming noise that developed on my car that would change pitch and tone with everything in the car including turning the steering slightly to using the window switches. I had various replies previously from members on another site last year who advised a change of power steering fluid to possibly correct the problem - this was done and it was still there. I eventually took the car to my BMW main dealers who had the car for a few days and agreed that they could here the noise, so a brand new genuine PAS pump, pulley, belt, and modified hose were fitted - but to no avail. The diagnosis was then given to be the 'characteristics of the engine' funny this, considering that this noise was never present for the first two weeks of owning the car. Anyway - i lived with this noise for all this time and i eventually got hacked off with it, so i tried to investigate it myself. Whilst driving the car, it sounds like the old austin metro (or a route master bus if your UK based) and then when idling - the normal persistant humming noise is there. So i got a friend to do two passes past me with both the clutch enagaed and disengaged - car sounds beautiful and quiet. Once my friend pulls up, i get inside and i hear the usual noise on idling and driving. Only the road noise covers it up. So i decided this was definatly something interior related (or can be heard inside) and i traced the exact same noise to around the steering cowling. I put a screwdriver against the plastic and my ear against the screwdriver, and that noise can be heard as though its coming from inside - also its confined to that area. before i start ripping it apart, does anyone have any clues as to what this is? Or what electrical (if it is electrical) is behind hear? or even if someone has had this problem.

Many Thanks guys
Dal