yeah I used to have the same sound from my 95 F-150, it had a 5 spd and a 4.9L6. Except it made the noise in every gear, it sounded like rocks in a blender.
I was told that it is the sound of the internal parts of the transmission vibrating or some other way making noise from the vibration created by the engine running at a low RPM, and is not casue for alarm.
