my Taurus sounds like a cow bell? need help

lately my 94' Ford Taurus has been making an interesting noise. It was only when it idled for a while, and now it is even when I feed it gas.
I took it to a shop to have it looked at , and of course it didn't make a sound.
It gives a knocking, it sounds like a cowbell. I know it isn't the bearings in the alternator, because I just replaced that. Does anyone have any advise?
Thanks

You may have a loose heat shield on the catylitic converter. (If there are any left on them by now) It would sound like it is coming from under the car. Removing the offending heat shield would end the noise, but not everyone would tell you that is the best idea.