Another way to listen for noise is to drive along a wall.
For example, try a wall that divides two split level streets.

As you drive close along it, open your driver side window and stick your ear out.
The engine noise is amplified. (drive along the wall, not into it!!!-DOH)

My 300D engine ticks sometmes. I have had the valves adjusted and new injectors recently.

Only heard if I really listen for it. This is not nailing noise, however.

By the way, check your trans fliuid level. It may be at mini-minimum.
While refilling a trans on these cars( After service-topping off), I heard a slight 'ticking'
until I got that pint of fluid in to mark the minimum level.
(car idling and in PARK)

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD