After a nice week of the van running sweetly, I developed a really
loud metal clanking sound in my exhaust system yesterday on the way
home from a lake.  The problem started at freeway speed and continued
at idle.  The metal clanking sound is so loud that it drowns out the
sound of the exhaust system. It is erratic and varies with engine
speed.    I drove the van for another 30 miles home with no adverse
effects.   I thought that maybe an exhaust valve broke off and was
rattling around in the exhaust pipes.  Or, maybe a piece of catalytic
convertor was caught in the muffler.  Sounds like a piece of metal is
caught in there and banging away really lound.  I'm not talking to
about a gentle, "tick, tick" or even a "clank, clank."  this is like
CLANK!!  TICK, tick,  CLANK!!, TICK!   Audible at thirty yards over
the sound of the engine.

I took off the U-shaped pipe that connects the header collector to the
catalytic converter.  There was no metal in there.  The cat side that
is facing the O2 sensor looks solid.  I haven't disconnected the cat
from the muffler yet.  I took the valve covers off and valves appear
to be moving.

For about 1 year, I had a malfunctioning lifter on exhaust valve on
cylinder 3. I now believe that this caused a range of problems,
including a ruined O2 sensor.  (two different mechanics in my area
didn't diagnose the problem correctly and I didn't have enough
knowledge).  After I replaced the lifter, it wasn't adjusted properly.
 I just adjusted it properly about 2 weeks ago.  I was thinking that
maybe something broke.  If an exhaust valve breaks, what are the
symptoms?  Again, I had adequate power on the way home.

I have a spare exhaust system and almost near cylinder head.  Right
now, I plan to swap in a used U-pipe, cat, and muffler combo and see
if that solves the problem.  If that doesn't work, I'll ask more
questions.  If I have to, I can swap out the cylinder head too.  I
don't want to go through that hassle, but I a am prepared to do it
this weekend if it will get my van on the road.

The van is on jack stands again.  At least it got us home.  It somehow
always gets us back.  :-)

Any advice on the sound would be appreciated.