Nissan: NissTech, help w/mysterious noise?

On Saturday I heard an ominous noise when I was driving my
1994, and for the life of me can't figure what made the sound.

It was very cold here (near zero including wind chill), and
shortly after starting the Pathfinder I was driving it up a hill,
with overdrive off.

I noticed that the power was quite poor, and a few seconds
later I heard a loud groaning noise. The nois seemed to be
independent of the movement of the car. Also, as a test when I
was stopped at a light, I shifted the car into neutral and I do
not believe that had an effect on the noise.

It seemed that the sound came from the left-hand side of the engine
area, looking from the driver's point of view.

While I was stopped at the light, the groaning sound began to
"wind down" somewhat like an airplane engine. Then the noise
stopped with a soft bump and all was normal with respect to
engine strength. Drove the car on a 50-mile trip after that
and it was fine.

I hate to bring cars into shops with vague descriptions, as
that can lead to an expensive fishing expedition, so any leads
you can give me would be greatly appreciated. Does this sound
like a bad timing chain? Near-frozen engine oil?

Note: Occasionlly I have noticed a different noise coming from the
engine area. It's a faint, high-pitched sound that seems to be
independent of the speed of the car. It stops after a few seconds.

Thanks,
Al

Update: in 2-18-04 posting below, I found that the noise
may have been related to the water pump or timing belt.
(Fingers crossed) it may be fixed now.