About the Mantra: With my discussion on "Why Gayatri Mantra restricted by Vishwamitra, so that other can not use?" it is said that there lots of thing in this mantras than just peace and good health. Mantra can also be used as Weapon, Mantra can be used for bad thing like for MARAN vidya. So, many of the mantras should must be kept in secret so that can not used for harmful purpose.
If we talk about Vedas mantra than, these mantra should be pronounce at appropriate time and with appropriate RAGA otherwise, you can not get vibration of mantra and may be you lost faith in Mantra.
And in last, on my discussion of above question of Gayrti Mantra, one sadhu(bava) told me that there is a reverse spell in Gaytri Mantra, which is related with BRAMASHTRA weapon.