A friend of mine wants to build an AR-15 in .300 for hog hunting.  He is concerned about the noise a typical AR-15 lower makes when you switch it from safe to fire.  It does make an audible clicking sound.  He is concerned it will frighten the game he is hunting. Does any manufacturer make a quiet type safety?  Thank you.

 
They are all going to click because you are hearing the detent falling into the pocket (probably the wrong term) on the selector.


He will likely be fine, animals don't spook and run off at the sound of something like that typically.  It's still quieter than stepping on a dry twig, and that usually causes animals to freeze.  

take out the selector detent and file the tip.  if he rounds it out a little, that will make it less distinct.  also take a few coils off the spring and there won't be so much pressure on it.  if he messes either up, it's a cheap $2 fix in new parts.

Use two fingers to "pinch" the safety down slowly if your paranoid about noise.
After a couple of range sessions all mine seem to smooth out and make less noise when I'm throwing them off with my thumb.

pack a little grease in it as well  

/\ This /\, or a few drops of CLP in the spring & detent...file down the detent just a bit too.

Didn't think about that, but he's right.  Just taking just a bit of the "pointiness" off of the tip will help some as will removing a few coils off of the spring to lessen some of the pressure on the detent.  You have to be careful doing this though, take off to much and your selector will get bumped off of safe quite easily thus making the rifle unsafe in the field.  



As another poster mentioned you can control the selector deliberately.  Flicking it will always be noise, but if you guide it carefully it can be pretty quiet.  

 

This.

All the suggestions are good so far.

What I do to avoid the click while hunting is ride the safety. It's much easier to do with a short throw ambi selector in my opinion. I use the v7 and Battle Arms but there are a few others on the market as well.

Add a bit of grease on the detent and spring. I have a BAD-CASS safety on one of my AR's, safety works perfectly as it should but makes almost no noise.

Also consider the better the quality of the machining the smother the operation will be.

I know this isn't really helpful, now, but after having my rifle cerokoted, the paint made my safety selector hole slightly smaller, which increases the tension on the selector.  I could actually run my rifle without the detent.  I have found the tension to be extremely annoying.  But, for your friends specific situation, my rifle would actually work very well.  But, you would have to make sure you don't push the selector passed the "fire" point.  My rifle doesn't have the external stopping points.

Over the years I have trained myself to take the safety off only after I get a sight picture, and put it back on as soon as I lose that sight picture. Except when still- hunting.  When on a sit, as soon as I become aware of potential game I take the safety off-with my trigger finger- and leave my finger on the safety until I shoot or put the safety back on.  Taking the safety off earlier lessens the chance that a deer would hear it.  If I was worried, though, I would consider breaking the corner on the safety crossbolt that the detent engages- not sure of the terminology but I think you know what I mean. If you leave the engagement surfaces of the detent and safety the same shape, function (and safety) shouldn't be compromised, but the transition should be smoother.

I would try to make a safety detent out of delrin if I was worried about it.