Metallic chattering sound - Mazda B2500 (Ford Ranger)

Got a problem with my 1999 Mazda B2500 (Ford Ranger under the hood), and
before I take it to my "plug it into the computer, no error codes, nothing's
wrong with it" mechanic, I wanted to run it by some people that know a
little more about vehicles. Here's the basics of my truck & the problem

1999 Mazda B2500
4-cylinder engine
5-speed manual transmission
130,000 miles +/-, original everything (including clutch)
2-wheel / rear wheel drive

Problem: vehicle makes a metallic chattering sound (like someone with steel
dentures chattering their teeth in the cold, best way I can describe it, or
maybe like a dime rattling around in an empty beer can) intermittently.
Sound seems to be coming from under the floor boards, close to the back end
of the vehicle. Sound is worst when there is a load on the engine,
especially in higher gears & when going up hill, or if I let the clutch out
too slow when up shifting to a higher gear. Sound only happened once when
there was no load on the engine. Engine sounds are normal, engine idles
within normal parameters (500-750rpms), accelerates well both on flats and
up hill. No idiot lights have come up on my panels (and this truck is like
a nervous child, if it thinks even the slightest thing MAY be wrong, my dash
lights up like a Christmas tree). Sound happens more often at lower RPMs
(just before I down shift) or when accelerating suddenly.

I had chalked it up to an old vehicle with a loose panel or something else
that's pointless to worry about, until today when a friend (and former
mechanic) said it may be something in the driveshaft and/or rear
differential. At first I thought the worst this could be was that my
130,000 mile clutch is finally giving out on me, even though it feels ok.
When he mentioned this, I had nightmare images of a bearing seizing up and
my transmission blowing out as a result.

Obviously to diagnose a problem, it helps to be IN the vehicle, but anybody
want to shed some light on the above? Feel free to ask for more info. I'm
just trying to get some feedback & suggestions before I take Chicken
Little's advice about the sky (and my driveshaft) falling.

C.