Odd Boxster Sound?

[JCL12]

Can anybody identify a potential source of this sound? Description:

-Car is 2001 tip box
-Appears to come from behind me on the left side (driver side)
-Sounds like a very slight rattle or something is loose and gently vibrating or grinding
-Sound volume is very low - I cannot hear it at all if I have the crummy radio at even a moderate volume
-Only happens when the car is not warmed up (If I'm at 180, I'll never hear it)
-Only happens at low RPM (<1600)
-Seems to only happen in 4 and 5th gear (and maybe 3rd)

I assume this is nothing, but I am curious!