Tapping noise

2004 sebring gtc , 55,000 miles, , front struts replaced 08/2007, new front tires 12/2007. it has been making noise on and off for a year. in august 2008 i brought it back to the dealer for the same noise. 3 mechanics drove it. it didn’t knock or tap for them. they checked the breaks and the suspension which are OK.the tapping only happens when the wheels are in motion. what could it be

i think is motor mount i had same problem but it will on make noise while in gear if you put in neutral and if you don’t hear tapping it’s motor mount