Problems with my 98 Grand Marquis?

i have never had problems with it. just purchased it with 89K on it a couple weeks ago and now it overheated on me today. The first time i was ten minutes away from the house so i drove it to my house with the ac off. I noticed that there was no cap on the overflow so then i drove it to auto zone and got a new cap and put it on when it overheated on me when i got to my house after that i have left it parked and let it cool down but now it sounds like a Diesel engine instead of its usual quiet sounding engine so my question is what should i do???