If it is quiet, it is a "D" turbo! There is no mistaking the sound of a "C". Joel had it right. Just picture the loudest turbo whine you have heard on a HD rig, multiply it by a factor of 10, and you have a "C" turbo sound.
Attached is a not too great picture of the "C" turbo on my Deuce. You can see the bolt that clamps the intake and exhaust sides together in the photo. Also note how the exhaust (left) side tapers in slightly as you go from bottom to top. On a "D"turbo the exhaust side is much wider and does not seem to taper.