hello everyone, here's my problem of this week (81 adventurewagen):

a chirping noise, only when engine has warmed (after driving a few
minutes).  the noise is most apparent at lower rpms in 1st and 2nd, but i
can sometimes hear it in 3rd and at higher rpms in 1st and 2nd. i have not
heard it in 4th.  the noise is not apparent when the van is not in gear, i.e.,
revving in neutral does not produce it.  occasionally the noise will be
present for a split second when the engine first starts.  after i have been
cruising on the highway for awhile, the noise disappears (won't occur at
lower gears/rpms), and sometimes reappears (at lower gears/rpms). 
fwiw, a compression test 6,000 miles ago revealed 110-125 psi on all
four cylinders.  also, i have been using a lighter weight oil (10w-30)
cause that's what was in it when i got it and i'm just using what was left
of a case :-0.  (next oil change gets 20w-50).

i originally thought it was an exhaust leak, but that doesn't explain the
intermittent nature, unless i'm missing something.  

i was planning on finally taking my summer vacation next week, and
would like to figure this out/take care of it if i need to before then.  any
suggestions on what to check out would be appreciated.

TIA, matthew