e38 - Engine tick / tappet problem??

Hi
I have a 2000 740.

I have owned 2 previous 740s, and a 745Li.

None of my previous cars have had this behaviour that my latest 740 has
developed.
At idle the car seems to 'tap / tick'. This is noticable up until about
2,500 rpm, where the noise gets drowned out.
I had the car at my local dealership last week, and they said that they
couldn't hear anything wrong. However, I am now convinced that the 'tick'
shouldn't be there. A friend of mine is a mechanic for SAAB, and thinks it
may be one of the hydraulic tappets... (?)

I can particularly hear the noise when the climate unit is off (ie. no other
fans blocking the noise), and sat in the drivers seat with the window down.
I've listened with my head under the bonnet, and it definately seems like
there is a tap from the left hand bank of 4 cylinders (although it may be
something else). However, the noise is definately from the left hand side of
the car when facing it (ie. the right hand side, when sat in it).

Something which may (or may not) be linked, is that there is an 'oil spot'
on one of the cylinder head bolts, on the left hand bank (ie. the noisy
side)

Has anyone else had this problem?? Can you suggest what it is??
My BMW warranty expires on Sunday!!! but the car is booked back in to the
dealership tomorrow, so I am going to insist they look again (as they said
that nothing was wrong when the car was there last week).

Any help / advice appreciated.

Thanks