Rattling noise only at operating temperature then rough idle then shuts off

Hello and thanks in advance. My car is making like a rattling sound but only when it gets up to operating temperature and only when driving. After about 20-30 minutes of driving when you’re stopped at a stop light or stop sign it shuts off and then it won’t fire back up until it’s cooled down. The water pump went out about a month ago and I was referred to using the Bar’s stop leak for cooling systems since I can’t afford the part seems like it worked well considering there isn’t any leaking anywhere. Any suggestions? I’ve replaced the spark plugs and the battery is maybe 2 months old and the voltage is good I had it tested and the alternator as well was tested also there’s no check engine light. Thanks again.