When running the engine has a squeaking sound

If the belt is off and it still, does it then you are correct that this is most likely a valve train issue.

However, this is normally caused by metal to metal. So that means we need to start with checking the oil and seeing if there is metal in the pan and filter.

https://www.2carpros.com/articles/how-to-change-engine-oil-and-filter

If there is metal, then we are going to have to get serious about tearing the engine down and rebuilding it or replacing it.

Basically, once there is metal in the engine then it is going to be in every bearing and passage so replacement or complete rebuild is the only real option to fix it correctly.

Here is a guide that talks about the other possible causes but unfortunately, I think we are dealing with the engine issue.

https://www.2carpros.com/articles/engine-noises

Let me know what you find with this, and we can go from there. Thanks