01 Pathfinder Manual Trans. Noise???

I would appreciate any advise on the following.

My 2001 Pathfinder (67K miles) has begun making a chattering noise
about 2 seconds after I let out the clutch and just as I begin to
depress the gas. The noise lasts for about 2 seconds and only chatters
after shifting into gears 1-3. I can hear it only when the window is
down.

Previously, I owned a 86 mazda B2000 which made the exact same chatter
noise for about a year before I got rid of it. I would like to have an
idea about what the sound may be before going to the Nissan service
dept.

I have an extended warranty through Nissan which covers.....

"Transmission case(s) and all internal parts including the torque
converter, vacuum modulator, electric control units, oil pan, oil
cooler, transmission mounts, transfer case and
all internal parts, transfer control unit, seals and gaskets, clutch
master cylinder, operating cylinder and booster, and shiftlock system"

I do not trust the Nissan dealership service department because they
seem to always "find" something that happens to not be covered under
the service agreement.

Does anyone know what this sound could be? And is there any heads up
as to what Nissan may B.S. me by telling me it is that is not covered.

Thanks!