Trouble is people confuse 'chatter' as found on external waste gates with compressor surge or stall as found in this case. You'll get both if the dump valve either cant keep up with the amount of boost thats trying to escape thru it or as said previously, if the spring is quite hard. In theory, compressor stalling is very bad and on high boost cars it can shatter an impellor shaft...altho no doubt many people on here will now say they've done it since their old man first started working down the pit and it aint blown yet blah blah lol