I have 1998 dodge stratus with a 2.5 engine that makes a loud rattling noise upon start up but then goes away immediately. It runs good for abot 20-30 minutes and then the engine starts to get hot (but not fully over heating) and then a valve train ratting type noise starts. I was told by a couple people that it was the oil pump, but after removing the oil pump and examining it did not appear to be worn or defective. Is it the oil pump or is it something else? Any help is greatly appreciated!