In the three 603s I've owned I've noticed a bit of a lifter noise at idle as they've aged. It comes and goes. Maybe/probably they're not getting consistent oil at idle; not sure. The second I bump the rpm to above 800 or 900 away the noise goes. I cannot see how if you have the updated vacuum pump that it could make noise they seem well built to me. I would look at the belt tensioner and spring and shock. That is a real weak point on the 603 and needs changed out more frequently than you'd think especially with some of the aftermarket crap that's out there.
I do advocate the stethoscope method. They are cheap and surprisingly will let you hear very specific areas of the running engine.

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DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV