I've not noticed this in my three CST743E's, but I'm not sure what you mean by the sound.
Which fill valve arrangement do you have: Type A or Type B (see the manual, page 4 for a diagram.) If you were able to adjust level easily without removing a white tamper key, then you must have the A which is a screw type adjustment.
Where is the sound coming from: Bowl? Tank? Fill valve? Flush valve?
You could probably determine whether or not it is fill valve related by shutting off the water supply to the toilet, then flushing. If you didn't hear it then that would mean the sound was coming from the fill valve. When you say the water pressure is high, how high?