I bought a 300 5.7 hemi. It has a hole somewhere in the exhaust. I suspect flex pipe. Old owner said he's been driving it like that for a while. It's really Loud! Will it cause a burned valve? I will replace with magnaflow cat back but I don't know if the damage has been done already. It doesnt misfire and there is no check engine light but I'm a worrier. I worry.
For the most part the answer is "no". I wrote an answer about burnt valves here (but also check the other answers as well). For the most part, burnt valves are caused by the valve not sealing correctly. An exhaust leak causing a burnt valve comes from the idea of reduced back pressure in the exhaust system allowing unburnt fuel past the valve (evacuating the cylinder prior to complete burn). This is really a fallacy. There is still back pressure in the system. Even engines running open headers or even zoomies (straight single pipe per cylinder) still survive without burnt valves, and they have far less exhaust restriction than what you are experiencing from your exhaust leak. Still, though, get it fixed as soon as possible. Exhaust gasses can cause other issues, like CO poisoning from it coming up into your passenger cabin.