Nissan: Help with mysterious noise

A couple of months ago, while I was driving my 1994 Pathfinder
slowly in a parking lot, I noticed a strange noise coming from
the engine area. It was a soft, high-pitched sound that seemed
to be independent of the speed of the car. It stopped after a
few seconds.

I've heard this same noise just a few times since then, but
yesterday I heard a more ominous noise that may be related to
the original one. It was very cold here (near zero including
wind chill), and shortly after starting the Pathfinder I was
driving it up a hill, in overdrive.

I noticed that the power was quite poor, and a few seconds
later I heard a groaning noise that was much louder than the
original noise I heard. The noise again 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.

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 torque converter? Timing chain?

(I should tell you that we had the transmission replaced a few
years ago after a shop misdiagnosed a fluid leak. They told
us we had an oil leak when it was transmission fluid, and the
transmission soon locked up and had to be replaced.)

Thanks,
Al