Eric,

  If a rod bearing is suspect I would drain the oil into
something that will strain out any metal bits that might be
present. Also I would try and open the oil filter and
inspect the material. You can not use a hacksaw because this
would create filings. I have used a giant pipe cutter.
  If nothing is present I would use a long screw driver to
home in on the noise. Place the blade on the block and the
handle against your ear.
   I have heard exhaust leaks near the head sound similar to
the noise you describe. Use a piece of hose as a Stethoscope
to locate these.
Good Luck

      Ken Lewis
 86 Transporter Crew Cab, 60 T-5 Coupe
 http://neksiwel.20m.com/   O2  sensor monitor gadget, my
cars, Vanagon Tech
 http://EB03.20m.com/  Everybus 2003  pictures