The answer to 'supply line constantly running' is that the most common cause is a problem with the toilet fill valve or flapper. This can happen due to a worn or broken part, or a problem with the water pressure or level in the tank. The symptoms are a continuous flow of water into the toilet bowl, which can waste a lot of water and increase your utility bills.
In many cases, this is a simple DIY fix that just requires replacing the fill valve or flapper. However, if you're not comfortable doing the repair yourself or the issue is more complex, it's best to call a professional plumber to diagnose and fix the problem. They can ensure the issue is properly resolved and prevent further water waste or damage to your plumbing system.