found what was knocking on my 06WRX
it is a black box 6''x6''x2.5 with pipes .5 dia connected to it
held in with 2 bolts (one missing) and rubber mounts
on drivers side by back bump-per
been looking for some time we thought that it was my BC coilovers
Not sure what it is?

 

I think there were a couple of other threads posted last week with the same question - and, IIRC, Splitpin said it was the carbon canister thingy...???

 

yes its the carbon canister.

 

thanks guys