Mystery knock/ tap

Video of noise:
https://youtu.be/Yj_dQpJYfi4

2000 miles ago, I changed my oil and it began to tick like this. I took it to a mechanic and he said it was the exhaust. I was inclined to believe him until it got a little bit worse. It only happens in drive, reverse or when the AC is on ( so only when there's a load on the engine ) at around 600( idle ) to 1000 rpm, then goes away. Has anyone else experienced anything like this. Took it to a mechanic again today and he said it sounded exactly like a lifter but was confused when it went away in neutral/ park. Thanks!

(Ps it used to go away when the engine warmed up but I added some STP synthetic oil additive recently to help and it pretty much happens all the time now)

 

do a compression test