Hi to all:

I just purchase a 1997 Nissan Altima with about 83,000 miles on it.
Maintainence records, clean oil, etc.  
I noticed when driving at about 2,700 to 3,000 rpm I hear what sounds
to be a rattle...They type that may come from a shield or guard. The
frequency of the rattle stays constant as you rev the engine past
2,700 rpm.  I've also listened under the hood, revving the engine by
hand, and it seems as though it's coming from under the engine.  I
must say though, that it is more noticeable inside the car than while
listening under the hood.  
The exhaust system was recently replaced.  When listening inside the
car, it seems as though it's coming from under the engine.  It also
sounds like it may be a bearing.  Does the idler wheel sometimes
produce this noise?  Could a frayed serpentine belt produce this
sound?  (Or whatever the belt is that rides against the idler pulley)

I stopped in my driveway, revved the engine up to the 2,700 rpm point
and hear the rattle.  I guess I can rule out the transmission.

Any ideas?  The guy that sold it to me just had the plugs changed,
changed the oil with Mobil 1 synthetic.  Other than the rattle, the
engine has lots of power, and I cannot see anything obvious->  I'm not
a mechanic though  :(

Thanks for the help.  I hope that it's nothing serious.

Art