I am a totoal newb!! I have had my truck for 4 years but have just recently began doing some minor DIY work. The other day I popped the hood to check my oil level and heard a hissing noise coming from back behind the battery near the firewall. Took a few minutes but I found that a hose that leads from the engine to a small box beind the battery it the culprit. In the meantime I have used electrical tape to patch up the leak but I want to find out what its for.