Sounds like oil not getting to one of the lifters at idle, something in the upper drive train. When rpm is raised making the noise seem to go away. The extra oil pressure dampens the slap sound.
My e34 made this sound every winter. I used esso xd 30 diesel oil one summer to clean out the engine and didn't make any noise the following winter.
Last edited by parm; 01-05-2010 at 11:28 AM.