Bill,
How many hours do you have on your tractor? If it's been doing this all along, and we're talking about the same noise, I'm pretty sure it's a normal sound. Your 4100 and my 4400 both have the wet disk brakes. When I first got mine, I took it back out to the dealer because of the noise the brakes made. Kind of a growling, groaning sound. He said it was normal and fired up a couple of other new 4000 series units to show me that they were the same. If your unit is a hydro, like mine, it will take some time before the brakes wear down a little and "smooth" out as you're not using them much with a hydro. If your unit isn't a hst and it still worries you, I'd arrange to have the dealer look at it and address your concerns.
Hoss