Incompressiblity in Fluid Flow

I am trying to model the flow in a tube where on either side of the tube there is a sinusoidal pressure being applied and in the middle on the side of the tube there is an outlet where the pressure is zero.
I need the fluid to be incompressible. However, when looking at the flows out of the model (surface integration at inlet/outlets with the magnitude of velocity) there is not a net-zero flow. Is this a common problem? Is there anyway I can correct this and make the fluid completely incompressible?

Thanks for any help!!