The sound is the reason some people call them "choofa's". Like you say, you get one good and hot, and it's beyond a simple "steam train" noise and more like a "purr."
One time, I put two stacks on one heater to see if it would make it draw better. It turned into a red hot jet that was so intense, we thought it was going to explode. I finally worked up enough courage to run up to it and attempt to shut it down, (closing the lid and closing vents on the filler cap). It started sucking air from every spot it could around the lid on top of the tank in a wheezing, gasping for life way.
Heavy, black smoke was pouring out of the vents and when it finally got enough air, it re-ignited itself with a "BOOM!" which blew the lid on the top stack off and a ball of flame shot out. It burned for two or three minutes and then started to smoke heavily again. This "BOOM!" ignite, smoke cycle went on for about an hour. I learned not to do that again.
