Blood rushing in veins
For the blood sound I use the sound of a sponge that I squeeze into water (with a hydrophone) to make the air come out, then I pitch down a bit. I would also add white noise under it to give it a bit of high.
– Rodrigue Amyot
Try putting a microphone into your mouth. You will get a nice low end sounding like blood. You will have to try to avoid making clicking sounds with your mouth.
– Morten Green