Despite the umlauts, Tantras of Gyütò are not heavy metal.
These two ethnographic recordings are example of a unique crossover. The people who tend to listen to them do not belong to the hardcore folk music crowd, but rather to a completely different audience - that of the minimalism, electronic avantgarde, krautrock, IDM etc. I guess this is a case of two extremes converging - if you wander far enough away from the conventional music, you might meet some people who left it heading in the opposite direction. And, indeed, these records are very far from conventional music, lacking melody, harmony, lyrics - they are in the realm of sound-painting that the modern man left some time ago and that some are now struggling to return to.
Centerafrique: Musique Gbaya/Chants a penser
192 kbps, 70 mb
You can get Tibetan Buddhism, Tantras of Gyütò: Mahakala at the extremely interesting ýlowek scavel-cronek blog, full of rare LP rips and other unique audio stuff.
Also worth a look: a small gallery of LP covers for african ethnographic recordings, mostly very beautiful.