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, 159K

Interesting:

On closer inspection and removing deposits from negative post this morning, I found that cell closest to post was slightly open. I could see slight discoloring from where open, and that may account for drip to and deposits on post.

i'm wondering: what effect if any does open cell have on the battery??????

Tangentially, Peter, the cat had been put on as a generic by Midas last September. you're right: could restrict exhaust flow, and then screw up engine. I'll monitor it.

Right now, i'm more curious about battery.

Time to pull out the ol digital multimeter. Let the car sit over night and without trying to start it and without putting the key on- check the voltage on the battery. If it is over 12.56 volts the battery is good. Take some black silicone and seal the crack in the top of the battery. Clean the negative terminal one last time. What I do to my terminals when I clean them is to coat them in vasoline - works great forever to prevent corrosion. Cheap and almost everyone has some. If the battery reads UNDER 12.56 volts after resting overnight - say 12.25 volts - it is weak and should be changed. If it reads 11.50 volts - or there abouts - you have a shorted cell and the battery is toast. After you have determined the battery is good, start the car. The voltage at the battery terminals should now be 13.32 volts- 13.89 volts at idle. At fast idle the voltage should climb to 13.67- 14.47 ( somewhere in that range depending on your alternator output). If your voltages are lower - change the regulator and brush pak on the alternator. My voltage tests may sound complicated - but is real simple and tells you exactly where you stand with your charging system.
Morris

Thanks. moedip.

Guess i'll make another small 9they add up) investment in the meter. Makes sense.

Can you answer the basic question I asked before about performance of batery when a cell cap had been left open?

Only thing that can happen is that some dust can get into the cell and the acid with splash out and start eating things. It shouldn't hurt the battery though. My 1930 Chevy was driven about 500 miles with the 6 volt battery strapped in place UNDER the car where everything under the sun gets tossed at it from the road. I thought I would check the water level in the battery and found that one cell cap was knocked off - probably from a stone hitting it real hard and was missing. That was 2 years ago. Put a new cap on it and the battery is still good. - in other words - don't sweat it. If you don't have a digital multimeter - get one - they are cheap and indispensable for checking for open wires, shorts etc etc. Once you get one you will wonder how you lived without it.!!
Morris