I can't find vibration!!

I have a low vibration sound coming from the front of my 1995 Windstar. It occurs at exactly 1400 RPMs. It stops at 1500 RPMs. It only occurs when car is in gear and moving. It happens in all gear ranges. If you put the car in neutral it will not happen. If you go to 1400 RPM with your foot on the brake it will not happen. Other than this vibration the car runs beautifully. I have searched high and low for something loose and have come up empty. Could it be somehow connected to the transmission or brakes? I am at a loss! Any suggestions would be appreciated.

check exhaust

So its just a vibration noise and not the engine stumbling... Can you feel it at all.?? If its just a noise something must be loose...

I would guess exhaust too. My 96 started to vibrate the sheild above the muffler was vibrating at cetain rpms. Also the 95s had problems with the torque converter. My friend had one and the bolts started to come loose. It was still under warranty at the time. I believe there is a tsb for it. Its worth checking.