After a history of engine temps hovering at the bottom of the LED w/ temps
in the 90's, our '87 GL has decided to slowly heat up.  Coolant tanks are
full and stable, belts are in good shape (recently replaced) and we're not
running the A/C.   What I have noticed lust recently (about the same at the
higher temps) is a heavy rattle which seems to be originating at the
muffler (which is rusty and sounded semi-perforated before).  The rattle
goes away when the engine is a little above idle.  In addition as the
engine heats (never overheated *yet*) up engine "power" & response drops
significantly.
My thought...The muffler is coming apart blocking the exaust flow. In turn
backpressure on the system keeping the cylinders from being evacuated
completely and progressively raising engine temperatures. Sound possible?
My other option is that the H2O pump is failing (because of a new belt??).

Thanks,  Chris A.
     '87 Vanagon GL
     '87 Honda Accord (201k)
     '86 Honda Civic Si (236k)