I got a 1998 Nissan Frontier 2 weeks ago.  It has an auto tranny and
110K.  It had the "Service Engine Soon" light on shortly after I got
it, and my mechanic fixed 2 items including the O2 sensor.  The light
went away after that.

The engine seems loud - at least, louder than a 2.4 4 cylinder should
be.  My mechanic has checked the truck out and has not found anything
obvious, including the exhaust system.  It seems louder on startup and
for the first 2 minutes or so after starting, but never gets really
quiet.   It's actually a buzzy sound (while driving, anyway), and this
buzz is transmitted to the cabin pretty strongly.  This makes driving
the truck enough of a chore that I won't keep it if I can't find the
problem.

Anyone have any insights about this?

Thanks, Steve