The metal murmurs featured in the gat sound library a classic example of spooky metal sound effects these are designed to be used as one shot stinger type sounds randomly throughout a spooky game level. This effect can be created relatively easily. The base sound for the effect is a bowed 14" cymbal, this doesn't have to be a cymbal but it is a good example.
The cymbal I have used is actually broken, it has a 12cm crack in the metal. This is good for two reasons. Firstly it gives it a slightly more abrasive tone when bowed, it also causes changes in the tone and resonant frequencies of the cymbal depending on the distance between the bow and the crack. The second reason is that it makes it the cymbal cheap, sites like ebay have a whole host of broken or damaged stuff like this that can be bought relatively cheaply.
Bowing the cymbal requires two things a bow and rosin. In this case I have used a cello bow as they are a bit tougher than a violin bow. Rosin is a solid resin produced from pine trees that is rubbed onto the bow hairs to create friction between the bow and the surface you are trying to bow.
The cymbal was recorded at 24bit 96k using a shotgun microphone and a contact microphone.
The shotgun microphone picks up the high frequencies nicely but lacks bottom end. The contact microphone is pretty much the opposite, it picks up the lower frequencies better than high frequencies. When mixed together this produces a much thicker sound.
To give the murmur some space a reverb is added, in the example a large church convolution reverb is used.
The next step is to add an echo effect with a large intensity this gives the murmur the "larger than life quality"
The murmur is then pitched down 25 semitones, this both not only lowers the pitch of the murmur but also stretches the length of the murmur greatly giving it an eerie drone like quality.