SRAM has an avid following for their higher-end grip shifters - pretty much all of the other companies have either ended production on them or limit them to lower end bikes. Personally I don't know if anyone else makes them but I don't know everything SRAM grips are known for that distinctive sound - I like it myself - there is no doubt how many gears I've shifted with that sound. To me it is a nice, solid sound and I like that.
I've XO grips on my custom road bike with the rest of the drive-train being a mixture of X7 and X9. My mountain bike is SRAM as well, with SRAM triggers plus a 10-speed drive-train (sadly they don't make grips for 10-speed yet, perhaps never). I don't know what might be going on with your cassette, but I've never heard anything from either of mine. I do typically coast at least part of the way downhill on the road bike and there is a good amount of coasting on my mountain bike. No noise from the cassette at all.
I've heard that SRAM has been trying to discontinue grip shifters since most of the market want triggers, but their grip shift fans are loud and vocal, hopefully they will eventually relent and produce them for 10-speed cassettes