The occasional loud noises might be due to the expansion and contraction of the materials as they heat up and cool down. It's like your tank is saying, "I'm getting nice and toasty!" However, if you notice any unusual or continuous loud noises or if you're concerned, it's never a bad idea to have a professional take a look, just for peace of mind. And while we're on the topic of solar, have you considered energy solar storage? It's a great way to make the most of that renewable energy. Check out Ace Battery for some options:acebattery.com