I have a new Trek with a 105 5700 Medium cage RD. When I'm on my 50t outer ring, I get noticeable noise (chatter?) from the pulleys. It's only on the 50t, not the 39t or 30t.
Is this normal? Any thoughts?
Well, 'cross-chaining' from your large chainring to your largest rear cogs is always going to be a little noisy- you should generally stick to perhaps the highest half of your cassette when in your largest chainring. If you intend to use the low gears on your cassette, move progressively into the smaller chainrings, as well.
You said this was a new bike, correct? You're very likely entitled to some sort of adjustment period. Buzz into your shop sometime and ask if they can take the chatter out- they should be happy to help.
First guess would be a very slightly bent derailer hanger. If the noise is only in the rear high gear (smallest cog) I'd guess a mis-adjusted high limit screw.
Definitely a bent hanger and check the pulley cage to make sure it is aligned. Chain wear is also a possiblity. Note if the chain jumps slightly as you go thru the gears, then, look for light between the chain and the cogs. As the chain comes around the rear sprocket, the chain will seat in the bottom between the teeth and as maximum force is applied to the cog, if there is any wear in the chain pins, the bushings will slide up just a fraction and give you that chatter. I usually ride 30+ year old junk and it happens to me around the 2500 mile mark on old suntour stuff. Good chance the pins and bushings are either worn, corroded or gummed up. It happens on my 55 tooth chain ring when its on the top two gears. If it is a newer bike, I would go for the hanger problem, first, then check the other things. It is never that noisy on the larger sprockets.