Hi,
  This is about a 96 Sentra  1.6 Liter engine with 170,000 miles.

  I've been  getting a loud rattling sound from the engine in all
gears, at high rpm's and light throttle, which started about 6 months
ago. For ex, if Ii put it in first gear (it's automatic) and take it up
to 20 mph, it won't rattle until I back off the gas and just give it
light throttle. Then it rattles loudly until I give it more gas or let
off the gas completely.  Does the same thing in 2nd gear, but at around
35 mph or so. The rattle sound is like a chain rubbing against metal,
or the teeth of a gear rubbing on something. Like the classic card in
the bicycle spokes, but at a higher rate. Like a loud buzzing.

The engine is often noisy at idle too, usually, but sometimes it nice
and quiet at idle, for no obvious reason. The noise at idle is a
clacking from the head, but not the buzzing sound. The clacking
continues when you put it in gear and step on the gas, but after the
rpms get up a bit, the clacking goes away.  Sometimes the engine is
quiet except for some low level valve noise, both when idling or
driving. Other times very clacky.

Oil pressure light comes on when hot, idling in drive, sometimes. I'm
going to switch to a heavier oil, to see what that does. Been using
10w-30.  Change oil every 4 to 7 K miles.

The buzzing is a different noise from the clacking, and I'm guessing
they are two different problems. The buzzing is always available, if I
drive it the 'right' way, but the clacking comes and goes.

Worn bearings ?    Rods?  Any ideas?

Thanks