S70 98 2-4L In-Line5 continues to emit an as yet unidentifiable noise
at left front end, only heard upon driving away when car is cold in a.m.
noise audible for typical 5-6 blocks then disappears.

it's sort of a shimmy (sp), high pitched noise that gets louder as the
speed of vehicle increases. don't know if it's suspension related or
some other origin.

have left with independent mechanic twice and he's finally heard it but
can't pinpoint it since it disappears quickly and only heard when vehicle
cold.

Sounds like the secondary air pump. It is usually a sign that the bearing in it are starting to go. www.matthewsvolvosite.com has instructions for a rebuild with non-volvo parts that is much cheaper than getting a new one. Also, water tends to fill the pump which is part of the reason for the bearings rusting and going bad. It's all on the site, so you can read the specifics. Just look under the "Volvo repair database."
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

There's a metal shield just behind the brake disk that can rub against the disk and make a sound like you describe. If you have recently had work done on the car, mechanic could have bent this shield. Easy fix is to take a large screwdriver and bend it back a little.