Will a bad tensioner sound like the motor is knocking & cause the car to stall?

It's unlikely this is due to a tensioner issue. I would not drive the car and get it looked at by a mobile mechanic / roadside assistance company, or have it towed to a shop for inspection. My guess without seeing or hearing the car is you might have a transmission issue...but it's impossible for me to tell so get it looked at by a professional. If you need a referral, look on this page for a RepairPal Certified Shop in your area. Good luck, Bret