Knocking on driver side low behind the ac unit

What is your question?

It sounds like a sticking valve in a short block chevy motor almost like its stuck but I don't know . It had a thermostat issue and the motor was swapped 3 months ago. I bought this car 3 weeks ago for my son it ran great til last night at the stop sign it bogged out then it would start but it would make that stuck valve sound then it would die. No fluids are on the ground. Do you think the thermostat caused it to overheat and its a bad internal problem anyone who might have insight on this please help