well mine has done it for about 6 years know and being the volvo mechanic I am I have checked everything. it is a garret on a 87 760 turbo and when I let off hard it goes warbwarbwarb and with help from a turbo manufacturer he says that it is called a sonic inbalance. Mine even become more noticable when I went from 12 psi to 17 psi however never no problems. But this does not mean dont check all the great ideas everyone has suggested becouse the bypass valve can make a noise but normally when that gasket is leaking bad it has a rough idle. good luck and happy boosting!