12 Hp Murray engine has noise that is similar to a bad water pump in a 1971 350 chevy

Hi, I have two 12 hp, 38" cut Murray riding mowers given to me by my father-in-law. One is 1989, the other 1996. They were thought non running. After quite some time of cleaning and carb rebuilding as well as oil changes due to leaking gas into engine. I was able to start and run both mowers. With the interchanging of tires and battery that was able to recharge from the 1996. I used Marvel mystery oil in the 1996, (which was the first to be restarted). Then drained it after several starts and just recently added an oil treatment to the engine, minus enough oil to have exact fill amount. The problem is both engines have a rattle in the lower part of the engine approximately in the area of the oil drain plug next to the starters. It is remeniscent to the sound of a bad water pump in an older carberated vehicle. What could be the cause of this rattle? The mowers did sit for several years and the 1989 had no plug in the motor during part of that time. I am very maticulous so these engines, even the mowers themselves; and the 1989 deck (which was in better shape) have been detailed thoroughly as well as mechanical reabilitated. Thanks for any help you can offer!

Don't know about Marvel mystery oil, and I don't know a Chevy water pump sound when it is going bad. I do know that small engine run at different temps and tolerances than cars. I would replace it with a straight 30 weight oil. The sounds you hear in the base area could possibly be valve rappers sticking or valves sticking. You have to realize that sound travels very easily in a sound engine and maybe caused in other parts of the engine.

Thanks for the suggestion I did use the recommended oil suggested in the motor manual. There are two mowers one I tried the Marvel mystery oil, the other; 1989 I used an STP oil treatment along witht the recommended oil and amount. The 89` noise is worse, remember now this motor sat longer and had no plug in it, but I did run oil through the motor and immediately drained it, doing this a couple times to remove any rusts or other debris that could have been in the motor from setting. I do understand these small engines run hotter, but for the time they both set not running it is a miracle they do run now, of course with a lot of work to get them to this condition! I just don`t want to run them too much without diagnosing the noise to prevent catastrophic failure in the engine.