Think the heat exchanger overheat sensor may be bad.
Before calling service technician, copy following links for troubleshoot resources:
http://waterheatertimer.org/pdf/Bosch_E9_troubleshooting.pdf
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
You should delime water heater once per year to prevent many common error codes.
http://waterheatertimer.org/pdf/Delime-tankless-water-heater.pdf
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7
SOURCE: Error code E9
High pitched noise is from a very high CO2 level. Adjust the gas pressure to stay above 11" w.c. and have a gas technician with a calibrated CO2 analyzer adjust the CO2 levels.
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