2004 Pontiac Grand Prix 3800 V6. Check engine light.

No transmission work prior to problem. Trouble code has been checked and is coolant intake/manifold related. 
I recently noticed a popping/grinding sound when I shift into reverse. I believe there are bulletins that note a harsh or garage shift into reverse,

but I haven't found anything about what causes it, if it's a precursor to major problems, how to fix it, or if it's simply a harmless characteristic of the transmission (which I think is a 4T65E). Will you please explain the details and offer a possible solution?!

Tech:

There are several service bulletins for this model year. Some TSB's may be related to your problem. None, however are related to the garage shift bulletin you are referring to. This would be for a very harsh shift into drive and  / or reverse.

These bulletins refer too many different conditions and possible parts failures. Anything from a reaction sun shell, 2nd clutch assembly, loose valve body bolt and blown out channel plate gasket, final drive gear, etc. You need to have a good Transmission Tech listen to it and check it out to prevent possible shop wanting to do a replacement when it may not be needed. Or at least change the filter and fluid, then look in the bottom of the pan for any dirt.

I would also have the check engine light cooling system code fixed first (don't know the code, it was not listed). Some check engine light codes can affect the trans. shifting. If the engine is seeing low coolant temperature, then it raise trans. line pressure and may cause a small problem to seem worse.