Very odd noise just started to appear on my 523.  The clank is happneing on idle (I don't dare rev it at the moment) and is intermitant every 5-15 seconds or so.  The noise sounds like a metal fan blade striking something.  It is coming for the air take side of the engine and appears to be between the 4th and 5th cylinder (from the front) and somewhere under, in, or around  the air intake manifold.  This is manual car with air con, and 90k miles on the clock.   Engine seems sweet enough, no warnig lights on and looking at the plugs they area all brown, dry and even.

I am a bit wary of driving it or even letting it run at the moment , any suggestions ??