Back into blower...
Tested weight in a few different positions. Finally removed it. No difference in vibration. (Put weight back in original position.)
Spinning blower slowly by hand, blower's outer rim is discovered off center by about 1/16." Bent it back as well as possible. Would have removed blower to check shaft but rivits retaining housing's teeth need to be drilled out to release cage --- lent drill out so there's no getting into cage today.
Fiddling around with motor... it's loose. On closer inspection, hard rubber standoffs on motor's retainer screws are worn down. So is plastic on cage where standoffs make contact. Fact that blower is not balanced (or not in balanced position), am thinking loose motor in cage might be making the noise.
Thinking about rotating rubbers to fresh position. Worn area is shinny spot near screw head, not large indent. (There's flat head screw driver slots in motor's retainer screws but they're useless when motor's in cage. Tiny adjustable wrench works here.)
Dropped rotation idea. Put metal standoffs on both sides. Will still look at shaft and possibly replace blower to eliminate vibration. Don't like idea of leaving metal standoffs on plastic without getting rid of whatever is causing vibration.
Vibration noise = gone.