Erroll Dietz is a remarkable fellow. He started out at National Semiconductor as Bob Pease's
technician, and rose to become Chief Technology Officer.
Feedback amplifiers generally have an output impedance that rises linearly with frequency---in other words, it's
inductive. As Dietz's short paper from Electronic Design shows, this effective inductance can
resonate with the output bypass capacitor to cause really nasty noise peaks in the 1-100 kHz region. If you have an inexplicable noise peak in that range, a small resistor (a few tenths of an ohm to an ohm or two) in series with the regulator output can be just the ticket.