When Hebb talks about "reverberation" in neural circuits, he still thinks in advance of our current knowledge of oscillatory neurodynamics. Here he speculates about the short term memory trace that holds position dynamically, prior to physical changes in the synapse:

"It might be supposed that the mnemonic trace is a lasting pattern of reverberatory activity without fixed locus like a cloud formation or eddies in a millpond"

From Hebb's 1948 "Organization of Behavior"