I have a rattle coming from where the transfer case is wont engage into 4x4 the push button light just flashes

check to see if you have automatic hub locking. These units can be vacuum or electric operated and are prone to failure of the system

ford ranger 1990
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Could be the servo on the front diff not activating. Could be a power prob or failed unit.

vacume problem for sure

Transfer case motor is probably sticking, try hitting the 4x4 low first, or if you can tap on it and try pressing 4x4 button should work.

Most probable cause is the engagement motor on the transfer case bolted on the back of the transfer case. Don't get it from Ford (too expensive), try Rockauto.com
In other cases the "4x4" light can go off when you push that button but the transfer case will still be engaged (still in 4x4).

It could be the switch, a fuse, a relay, the motor that engages the transfer case (very common problem) or the transfer case itself. Disconnect the motor arm from the transfer case and see if you can move the shift rod by hand. If you can then you know the transfer case is probably OK and you can test from the motor back to the switch.

check fluid level in transfer case and front axle

control module behind the passenger side kick panel

I had the same problem with the shift on the fly push-button 4x4 on my 97 Suburban. GM Shop manual indicates that it could be due to an encoder error on the actuator motor. Try pulling the transfer case fuse to reset the transfer case control module. Wait at least one minute. Re-install fuse then re-try operation. Did it last week and that saved me a trip to the dealership and extensive troubleshooting.