Ok, the Bosch, and the stocker are all designed to leak air under vac conditions.
When you go from vac to pressure over atmospheric, then the valve closes. Untill you close your throttle, then the vac pulls the valve open again, and boost pressure leaks back into the intake.
The flutter sound is just it working correctly. At the "zero" mark, where your just at vac to pressure, you can get these valves to make some pretty funny hoot owl noises. (I have two of them for more flow.)
Here are a few reasons they rock.
First, make sure you installed it right. They go with the vac hose nipple UP. Do not put them in sideways, they leak and do not work right this way. (However, they fit better, but don't be fooled, they do not work backwards.)
To be clear. Looking at them from the side, you have a port on the bottem, and a port out the side. Opposite the bottem port is the nipple for the vac line.
You want the port out the side to go to your IC pipe. The port out the bottem goes to your intake pipe.
Any other arrangement results in leaks and poor performance.
Ok, cool thing number two.
They leak! Yes, under vac, like when you step on the gas, and your not boosting yet, they are a SHORT CUT from the throttle body to your air filter. This is one of the reasons I run two of them. Improved low end throttle response. It's like you don't have a turbo, intercooler and yards of pipe to bother with, but a shorter intake. As soon as the boost comes in, they close up tight, and are held closed by the boost pressure. THEY CAN NOT LEAK UNDER PRESSURES WE SEE IN OUR CARS.
(Unless yours is defective, or broken.)
I have mine setup so they both blow the bypassed air straight at the turbo, helping to keep it spooled up in theory. But off boost, around town, they do improve throttle response I think. (Of course, my car has higher compression, and more displacment than most 7M's, but that's how I designed it.)
