[pippintook]

My 77 TA 400 engine just started making an unusual noise.

Background: I don't know the complete history of the engine. I believe the engine is stock and has not been apart. It has stock exhaust manifolds and mostly stock exhaust. The engine has ran fine for me the last 4 years. Generally, runs good and no issues.

A few days ago I started up and engine seemed to be making a light tapping
or popping sound. It appeared to be coming from the driver's side of the engine. I shut the engine down after maybe 1-2 minutes.

I started today and when the engine first started there was no noise. Note, the outside temp is around 35-40 degrees when started today. After engine ran for about 10 minutes I started to hear a noise. It seems to be coming from driver's side. I have recorded a few videos of the noise. Check these links - sorry video is a bit shakey

https://www.youtube.com/watch?v=h53eeTS7Vss
https://www.youtube.com/shorts/09MvYGGIW60

Does this sound like a lifter ? Exhaust ? Something else ? When I crawl under - both the drivers and passenger down pipes look ok. I sprayed some soapy water on the drivers exhaust manifold to down pipe connection and can not see any bubbles. I guess I should pull the driver's valve cover and inspect ? Maybe also pull the drivers exhaust heat shield and check the exhaust manifold ?

Any ideas or advice is appreciated.

Thx