I hope someone can help me with this.

Two years ago, I had a tuneup, new plugs and all, flushed fluids, new oil the
works.  Then drove on several 4 hour road trips.  I come back and begin to
notice that the car is making metal expansion sound or creaking sound like it
gets too hot.  The temp sensor looks good.  Also, my fuel milage has dropped
from 420 miles per tank to about 320 or 350.

I get new spark plugs. (yeah the BOSCH titaniums) and the fuel milage goes up
to about 380-400 per tank, but the creaking sounds remain.  I take it to a
couple of places.  The water and oil systems are good.  The dealer wants to
sell me a new engine, another says it's the transmission going bad.  

The transmission gets a fluid change with no problems noticed.

So last month, I put the Iridium NGK spark plugs (new) back in and the milage
drops to 320 a tank.

So I need to use this car more now.  Am I looking at thermo sensors, O2
Sensors, new(err... that thing that goes to the spark plug, ugh, brain dead,
core something) as problems

Worse, I wonder if I have lost a gasket.  I checked under hood, no oil leaks or
any other kind of leak.  There is a faint smell of gas, so I am going to check
to make sure all the plugs got seated.

I really want to start with the thermo sensor, but I hear that can be a pain,
so I am taking suggestions.

Thanks in advance.

Keith