Without a schematic showing the points you are monitoring it's not possible to say, but in general ringing occurs whenever there is inductance or capacitance or both with a current/voltage step (pulse) applied to them in parallel or in series (like you see in switching power supplies, by golly). It can be reduced by lowering the Q, such as by adding resistance, or reducing the rise time of the pulsed source that is hitting them. The frequency of the ringing can be changed by changing the amount of capacitance or inductance. It can also be caused by circuits or components (such as op amps) that react slowly to fast changes (under-damped). They overshoot their mark, come back and undershoot, then overshoot again, and so on, until they finally settle to a final value. It can also be caused by a mismatched impedance between a pulsed source and a load at the end the wires feeding the load. It can be difficult to quickly diagnose and eliminate. Sometimes it can be reduced by brute force filtering it out if doing so doesn't affect the desired parts of the signal adversely.