Please share your model so we can see what's causing it. Seems unusual. What is the longest pipe length elbow to elbow distance?. Usually for water hammer the axial mode is being excited. Is it Time history analysis? Usually 100 to 300 Hz would be enough for cut-off frequency. Is it long km pipeline?

What you can do if you have a Time History of WaterHammer Forces you can convert using DLF Spectrum Generator in Caesar II and check at what frequency the highest force will occur. Run a modal analysis of your piping system and check the natural frequency. Then from there you can compare what is the nearest frequency the piping system will be excited by your peak waterhammer frequency. Then you can use this as your basis for your frequency cut-off. Maybe add +50Hz?

Anyway, I'm also new to water hammer calc.

Any other opinion is highly appreciated.

Cheers!!!