Is your car on the original front engine mount? Do you find 2nd gear a bit hard to engage sometimes? If you were driving behind your car from a stop, and you watched the boxster's tailpipe when the person takes off, does the tailpipe dip alot? Do you have any slightly odd vibrations associated with engine speed?
Those are symptoms of a worn out front engine mount that can cause knocking sounds.
Next up is this: is the sound, when you hear it, getting slowly worse? Is it related to the rotational speed of the tires? If the sound happens faster when turning left quickly (i.e. at higher speed) and slower when you are moving more slowly, then it's almost certainly rotational in nature.
Rotating parts that cause knocking sounds are routinely related to the wheel bearings and CV joints. If you've ruled out CV, consider that you have a bad wheel bearing that only creates noise upon a specific load. Listen for a worn out bearing - if you can't hear it, consider jacking the car up (safely! with wheel chocks!) and seeing if the wheel has "play" in it from a loose bearing. It should not move back and forth on the rear - if it does, your wheel bearing may be loose.
No matter what, as a general rule of thumb, an occasional light knocking sound upon a certain load is going to be safe to drive on (there's always an exception)....as it gets worse, start to worry more, but also realize that it'll become easier to identify
