I had the same sound and I did a valve adjustment this past weekend just because it was past time to do it. It ended up making my engine twice as loud. I had a mechanical buddy check it and he said the valves were adjusted properly just that in general as cam shafts wear down their operation gets louder.
So now I'm deciding on whether or not to replace the whole assembly. Right now it's running alot better... just my little bimmer sounds like a F350 truck.