It is not normal. You are losing coolant somewhere.
If it is not leaking on the ground i would guess that you are burning it. And i am guessing that it is a head gasket leak that isn't full blown yet, meaning you only lose coolant when the engine is running because it is sucking it in. Once the head gasket goes all the way you will see it in the oil.
You will not see any smoke or water coming out the tail pipe when losing such small amounts of coolant due to the cats.
Sometimes on rare occasions if your PCV valve and/or entire PCV system is clogged/faulty the opposite pressure can cause the engine to suck coolant.
You also might want to make sure it isn't a leaky heater core and the coolant is running out of the condensate line (Where the air cond drips on the ground)
Any white smoke coming from the exhaust? This could indicate a head gasket leak. Have you pulled the plugs to take a look at them? If they're extremely clean, or if you notice coolant on them, again, could be a head gasket. Take a look at the timing cover area, especially if your 97 is a 4.2L.
Post your engine and mileage. I think we could all help you much easier if you do that. Good luck!
There were a couple of problems with the early versions of the 4.2L Ford motor. One is the head gasket leak, the other...
Mysterious Coolant Loss
The AERA Technical Committee offers the following information regarding coolant loss on 1996-2002 Ford 4.6L VIN 6, 9, W and X engines. Varied amounts of coolant loss have been reported on these engines. In some instances, no obvious external sign of the coolant has been seen.
Coolant loss may be traced to small cracks in the intake manifold coolant crossover. The coolant crossover is located below the first intake runner as shown in Figure 2.
Damaged intake manifolds should be replaced. Ford Motor Company offers eight different services consisting of new intake manifolds and all the necessary replacement items specific to individual Ford car or truck models.
I didn't get what motor you have but while the truck is running check the heater hose on the passenger top rear of the motor. I had one of those that only leaked when the motor was running once, quick fix with a flathead.