You still might try the look/see. If the noise is from the front of the engine, where the belts are, it could be a pump. If the noise is more from the top, it could be the hydraulic lifters, as this is where the gap will be audible. You could always get the local mechanic to lend a trained ear, too.
If it's making noises it's running
'96 E300D 58k mi (wife's daily ride)
'95 Audi 90 118k mi
'92 GMC Suburban 138k mi
'85 300SD 230k mi (my daily ride)