Have a tapping in front of engine could it be an actuator

It could be caused by the oil control valve, it's located on the side of the cylinder head, the vvti actuator is the gear on the front of the camshaft, if it was replaced try replacing the VVTi oil control valve. It can also be caused by a worn chain tensioner.

I would need to hear the noise to diagnose it.

Some vehicles have a actuator in the transfer case that the lever pushes against to engage 4x4.

There are actually two actuators, one is located in the transfer case and one on the front differential. the one on the transfer case provides power to both front and rear driveshafts, the front actuator must engage so that the front differential locks and provide traction to the front wheels. This means that even if the transfer case is engaged in 4x4 mode the front wheels will not have traction if the front diff actuator is not working.

the pump relay contacts are stuck together.unfortunately they are integral to the controller/actuator unit.$$$$$$$$$.you could try tapping on unit with small hammer to see if the vibration will separate them.

The 4x4 system on an 88-98 full size gmtruck is controlled electronically except the shifting of thetransfer case which is done with a lever. When the transfer case isshifted into a 4x4 gear a switch in the upper driver side of thetransfer case is closed this sends power to the front axle actuatorwhich engages the front wheels to the front axle. A sensor is alsolocated on the front axle which is closed when the actuator shiftsthe axle into gear.

The first thing to check is the 4x4fuse if it is bad replace it.
Next check the switch on the transfercase connect a continuity tester to the switch and shift the transfercase into 4x4 the stitch should close, if it dose not replace it ifthe switch is functioning properly replace actuator on front axle

take actuator back out then plug connector back on it but leave it out of axle now turn key on and shift into 4 wheel drive if actuator moves out then problem in in front axle u need to pull right half of axle apart if it doesnt move then it electrial problem to actuator check fuse

On the full size, the front actuator is electric, not vacuum.

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

Good luck. The actuator is usually the problem 90% of time but recheck all of the wire harness just to maker sure there is no short before replacing any parts.

Check that the heater tap is working, its located under the bonnet in one of the heater hoses, you can also manually move this tap.

If this works ok then the heater actuator inside is faulty. If any sign of leakage / stiffness rplace with new one. Common for them to fail.

Major work to replace this part.

the heater switches in the 4 runners can someetimes get jammed because of corrosion inside the tap itself the easyest way to see if its jammed is to locate it behind the engine down near the firewall you will see two hoses running through the wall and one of these will have a tap . looks like a plastic fitting with either a rod with an actuator aarm or vacumm line running to it . witht the engine up to running temperature and the heater on check the side of the tap that runs to the engine for heat . if is cold then the tap is jammed closed . if its hot then you may have a blocked heater core