Cat problem or ripoff?

I’ve been listening for about 1 1/2 years to this noise in my '99 Toyota Tacoma. Starting at 20 or 25 mph, it’s a thin, high rattle or swish, like tiny bits of metal or pebbles in a tin can, but it can sound like a little bird chirping, particularly when I’m lifting the accelerator, and it originates behind me. Numerous people have offered guesses or shrugs, one mechanic lubricated the rear differential. but there’s been no change in the sound, and no further trouble, in all this time. Then, last week, I began experiencing a hesitation or judder upon pressing the accelerator right after starting the truck. I couldn’t see how this could relate to the noise, but I took the truck in and described both problems. The mechanic kept the truck for a couple of days, cleaned the air filter (which has apparently cured the rough starting), and presented me with a diagnosis: air/fuel mix improperly calibrated, resulting in bad catalytic converter and mild back pressure resulting in lower performance, with little fragments of the cat lining bouncing around in the exhaust system causing the noise. Sounded plausible, but get this: the estimated cost of repair is $1725, with $1200 of that for a new cat and $290 for a mass air flow meter. Ouch. That sent me down to talk to he guys at NAPA, who found that cats for this truck actually cost between $150 and $275, and air flow meters around $110! The NAPA guys referred me to a local muffler shop, whose owner is skeptical of this whole thing - as am I. If the rattling noise started 1 1/2 years ago, and if it were actually what this mechanic says it is, wouldn’t my truck have been in serious trouble long before this? And if cleaning the air filter fixed the hesitant startup, and I’m not detecting any other problems - well, it seems to me I’m just back at square one. I still don’t know what the noise could be, but I DO know that I’m not about to pay for that mechanic’s Caribbean vacation! Am I thinking right about this?