someone posted this just the other week, I suggested looking at the air intake for a crack, leak, or loose fitting it will make that noise and cause some performance loss. That was his case, worth a look as its a quick fix.
I had the same problem on both my 98 GT and 98 Linc towncar. It's the IAC. you could soak it in carb cleaner and toothbrush the carbon clean, but that only lasts 3-4 mos. My noise is back on the GT. autozone or advance has them for around 50 bucks