I was on the freeway after a 2 hour 75mph trip but never exceeded 3k
RPM on the tack, when I got off, I had a ticking sound as if it were a
lifter that progressively got worse.  I'd had castrol 5-30 in the
engine from the winter and stopped by a lube outfit and had it changed
with fresh 10-30.  The noise appeared gone but came back after about 3
minutes.

It sounds like a woodpecker - sounds crazy but it sounds like either a
woodpecker or the rata-tat-tat of short machine gun bursts and almost
like it could be a spark plug wire arcing to ground.  Performance is
good though it *seems* like suddenly my fuel consumption is up too, in
terms of miles per gallon.  The sound/no sound of this is like you
hear if for a couple of seconds and then you don't for a couple of
seconds and then it's back.

Does this even sound remotely familiar to anyone?  Sounds like a
lifter type noise coming from the drivers front corner of the engine -
engine is a 3.0 v6.

thanks very much for any replies...

Chris