Some questions on 4x4 5.7 with 4L60

[gdocktor3]

I recently picked up a mint 95 Sierra k1500 5.7 auto with 152k miles. Orig owner up till 146k miles was a fanatic mechanic and replaced all known problems (brake system, fuel system, charging system) posi rear, dexcool and tranny flush every 50k miles plus more. Second owner barely drove it and also too good care of it. Never towed or beat on. Yesterday I was driving and in 1st gear I could hear a slight, short lasting clicking or chatter? for lack of better word, when going up hill. To better explain, when I first got the truck I did not know about the slow 2mph roll and firm shift into 4 low or neutral and instead tried going nice and easy which would grind and could hear that same clicking sound like something was spinning and not engaging. Not sure if it's related, but that's the similar sound if anyone is familiar with it. It goes away before shifting to 2nd and doesn't do in reverse, yet. When I put truck in 1st gear and went through rpms I couldn't hear it though. I've read it could be related to the planetary or sun gear. Is that about right?

My questions - Would a fluid change and filter help this?

Are the transfer case sounds related to this in any way?

Can the planetary be replaced without disassembling entire tranny?

Is there anything else I can do to determine actual problem?

If I do fluid/filter change, should I risk flushing entire thing or just do the pan fluid?

Any other info/advice is appreciated. I've owned lots of 4x4s, this being my first gm, but never had problem with tranny and/or transfer case. Right now this sound isn't affecting anything, but will it affect or destroy other parts of tranny if not replaced soon or can I wait until it completely goes? If I have to take entire tranny apart I'm going to do everything. Im a standard tranny kind of guy if you can tell by my lack of knowledge and this is a perfect reason why. Thanks for help.

[2018GMC]

Are you sure it is coming from the drivetrain? There is a heat shield on passenger firewall by exhaust manifold that can come loose and make ticking or rattling noise.

 

Also check for exhaust rubbing on crossmember or transfer case.

 

I assume this is the manual shift transfer case with shifter on the floor and not push button electronic?