hello, you certainly know, especially when the egr valve is blind, that when the rpm drops, the turbo makes a "stututu" sound. it is because the excess compressed sound cannot be blown out through the egr valve. I used the reverse flap and the problem with the sound disappeared and at the same time the flap does not let the exhaust gases into the intake. but the egr valve does not always try to release the compressed air into the exhaust. and then stututu is heard. but 80 percent less often.