White smoke when backing up, AOS?

[sasha055]

Hi,

Recently I started getting a lot of white smoke when backing up. It looks like the smoke comes from under drivers side rear wheel.
Is it AOS?

I did the oil cap test, no vacuum there, it doesn't smoke on start-up or driving, only when I'm backing up (from time to time).

There are no visible oil leaks.

Can it be AOS starting to go? Should I look at something else?

Car is 2000 Boxster base with 72K on it.

Thanks
Sasha