Thanks for all the info.
As a matter of fact, yes, the rust is on outside of the actual filter. It's held by a metal strip/clasp which doesn't have any rust on it (it's aluminum I think).
However, the filter is covered in rust. It just doesn't look right and, I think, might be contributing to the rough idling etc. I'm experiencing.
Also, the car is a '98 w/ 145,000km and I get the impression that it has never been changed.
