I doubt the battery cable corrosion would result in rough running. Rough running when wet would suggest old and tired spark ignition cables. Clean the corrosion anyhow.
Converter rattle could be a couple of things. First, make sure it isn't something as innocuous as a loose heat shield. If, after careful study, you find it's internal, it's a good bet the internal ceramic monolith is breaking up. This happens often. The problem is some of the pieces can lodge sideways restricting exhaust. This'll result in poor acceleration and loss of high end performance. It's not an urgent repair, but you may want to schedule an appointment in the near future that's convenient for you.
95 E320 Cabriolet, 131K