  This work describes the operation of a High Frequency Gravitational Wave
detector based on a cryogenic Bulk Acoustic Wave (BAW) cavity and reports
observation of rare events during 153 days of operation over two seperate
experimental runs (Run 1 and Run 2). In both Run 1 and Run 2 two modes were
simultaneously monitored. Across both runs, the 3rd overtone of the fast shear
mode (3B) operating at 5.506 MHz was monitored, while in Run 1 the second mode
was chosen to be the 5th OT of the slow shear mode (5C) operating at 8.392 MHz.
However, in Run 2 the second mode was selected to be closer in frequency to the
first mode, and chosen to be the 3rd overtone of the slow shear mode (3C)
operating at 4.993 MHz. Two strong events were observed as transients
responding to energy deposition within acoustic modes of the cavity. The first
event occurred during Run 1 on the 12/05/2019 (UTC), and was observed in the
5.506 MHz mode, while the second mode at 8.392 MHz observed no event. During
Run 2, a second event occurred on the 27/11/2019(UTC) and was observed by both
modes. Timing of the events were checked against available environmental
observations as well as data from other detectors. Various possibilities
explaining the origins of the events are discussed.
