The Howler Alarm works well for telling you when your nuclear reactor is overheating (with a heat sensor) instead of building the more expensive Industrial Alarm which works better for intruders.
This item can be disabled by server mods - it can still be crafted, but you will not be able to place it. (At least this is the problem I find)
To add your own sound, put it to the ".techniclauncher/tekkit/resources/newsound/ic2nuclearControl" folder in your appdata folder. Sound should be in ogg format and it's filename should start with "alarm-".
WARNING: If placed next to an active Redstone torch or lever (for a long period of time), your game is likely to crash.
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