Its a pretty safe bet that your idler and tensioner pullys are at the end of their life expectancy, I just changed them on my '98 windstar, and on my '90 bronco. Both vehicles have about 125,000 miles on them.
Now, I am not perfectly clear on the air pump, but I believe that it will sometimes pump air into the exhaust to further oxidize CO into CO2 in the Cat, and when the extra air isn't needed, the air is diverted out through that little muffler. So my question is this... when you checked that little muffler, was air getting pumped out of it? As a side note, last month I replaced my air pump because it seized. A couple of days previously, I had been mudding, and vigorously hosed down the engine compartment. When I went to get a new one, they guy behind the counter stated that if one gets moisture inside an air pump that they will seize. That didn't make sense to me, but it did fit my scenario.