The RDDRONE-FMUK66 schematics, gerbers etc are here: https://www.nxp.com/design/designs/px4-robotic-drone-fmu-rddrone-fmuk66:RDDRONE-FMUK66?&tid=vanRDDRO.... Generally you should not back power the FMU with 5V externally. the signal lines are all 3V, so it could be possible to damage something. But there's nothing obvious as to why the GPS beeper would suffer.
The RDDRONE-FMUK66 schematics, gerbers etc are here: https://www.nxp.com/design/designs/px4-robotic-drone-fmu-rddrone-fmuk66:RDDRONE-FMUK66?&tid=vanRDDRO.... Generally you should not back power the FMU with 5V externally. the signal lines are all 3V, so it could be possible to damage something. But there's nothing obvious as to why the GPS beeper would suffer.