mystery steering vibration

[swampy]

2003 SSEi 71K: At low speeds, especially when backing or moving forward slowly, the steering wheel vibrates lightly and a rubbing sound lasting for just a second or two can be heard somewhere forward (sounds like under the hood but can't be sure)-seems to be on the left side. It is intermittent and does not effect performance just a little irritating because I like everything to work perfectly. Here's what my mechanic (who is really good ASE gearhead) has tried:replaced rack with rebuilt GM rack, new steering pump, synthetic purple steering fluid, switching left and right steering components and switching them back. Still making the vibration and sound. He's baffled. I have a 2000 Chevy truck with about 170k which does a similar thing but it is worse.

[bobgto65]

I assume he checked the wheel bearings and CV joints?