I have blacked out from this... once. I stood up, felt the typical dizziness, heard the noise, and my vision was narrowing. All stuff I had started experiencing pretty regularly since I started seriously training. I grabbed the doorframe to steady myself, and the next thing I knew my wife was looking down at me asking "What are you doing?". I looked around, realized I was on the floor (with a bump on my head), and replied intelligently "I don't know." Interestingly, that's the same response I've given when I've crashed and someone has asked if I'm okay.

I saw my cardiologist, and had several tests, including echo-cardiagrams and wearing a Holter monitor to record my HR for 24 hours straight. He ended up referring back to a nuclear stress test he had already conducted, saying that, with no indications of anything bad in the additional tests, it was the best indicator there was of my cardiovascular health. I was fine, and just needed to avoid blacking out and hitting my head! Tensing your muscles briefly and waiting a few seconds before standing up can help. When you have the more serious symptoms (noise and the narrowing of your vision) my advice is to get close to floor. :-)