A: Air in the system leads to noise. The entire system must be inspected and bled.
B: The pumps may be oversized for the system in question. To fix this, throttle the pressure-side valves until the noises are eliminated.
C: The water in the system is boiling because the pump is undersized. This can be remedied by installing a larger pump.
D Excessive wear of bearings (rattling). The pump needs to be replaced.
S: A system that has already been entered into operation has become clogged. The system needs to be cleaned.