The Calculator application made by HTC, included with the HTC HD2 and several newer ROMs features haptic feedback on key presses within the application which you can easily disable or customize to your liking.

In order to disable haptic feedback in HTC Calculator, you need to edit your registry ar HKLMSoftwareHTCCalculator and change the value of Vibrate from 1 to 0.

For customizing the strenght of haptic feedback upon key presses (works only if Vibrate is set to 1), go to HKLMSoftwareHTCCalculator within your device’s registry and edit VibrateLevel to either one value from 1 to 6 (1 being the softest and 6 being the strongest level of vibration).

In order for changes to take effect, you either need to Soft Reset of end the Calculator task from any Task Manager you use.

(Source: XDA Developers)