That rumbling/rattling noise you hear from your pump is the symptom for any number of potential problems. The first step is identifying the problem. This article will help you determine what exactly is wrong with your pump.
The Hydraulic Institute has launched a new National Pump Systems Educational Initiative called "Pump Systems Matter." Its primary focus is on pump systems education and outreach as well as energy savings.
Install a flow meter in the recycle, cascade the flow set point with a pressure control loop. With proper tuning, the valve characteristics are lost in the flow control loop, which should be tuned fast with PI.