They are 100,000 mile spark plugs. I change mine at 50,000.. Make sure you use ACDelco plugs. No matter what car make you have, its always best to use the same brand plugs the manufacturer used originally.
The grumble noise.... does it happen when you raise the engine RPM slightly? See if turning your AC on and off. If the noise is only there with AC on, then its probably the same noise as mine. I've thought about installing an AC inline muffler, but its not that big of a deal to me since I narrowed it down to just a line noise.