Pulsations in a twin duct

Confused

Dear All,

I am calculating the flow from a fan that distributes air to two ducts. I had some convergence problems or strange residuals. I have therefore calculated the flow as transient now. I can see that there are some large variation in the mass flow to each duct as a function of time.

Is it always like that and can it be avoided? How to determine when it will happen?

Thanks in advance