So my heater is rated for a 29 gallon tank. I have it on a 20 long right now. Everynow and then it will make a hissing sound. Its only just almost a year old. It works great, always set less then medium (usually at low). But everynow and then i will notice that for a day or so it will be fine, then every 20 seconds ill hear a slight hiss. Then i turn off the heater let it sit, turn it back on later and it stops the sound. Any theories on what this could be? Its a good heater, id hate to have to buy a new one.