240d engine knocking after reconnection thermo vaccum valve

[domlemart]

While trying to figure out why my engine won't shut off when I turn the key, I noticed while poking around under the hood that both lines going into the thermo vacuum valve were disconnected. I reconnected them, and the engine would crank, but not start. I went back and disconnected them again, and the engine fired right up - unfortunately, I noticed a slight knocking sound that I'm pretty sure was not there before... any ideas what this could be? I'm new to this car and pretty new to working on cars in general so any advice is very very welcome because this does have me somewhat concerned.

[Diesel911]

What does the Thermo Vacuum Valve go to?

On my 300D the Vacuum That Controls the EGR by way of the Thermo Vacuum Valve goes through 2 sets of little plastic valves that are actuated by a cam plate and that mess is located on the Valve Cover.
The plastic valve arms wear and the O-rings inside of the valve can rot out ant likely other issues that might cause a vacuum leak there.
But, I don't know what your year and model has.

If you have an EGR the EGR diaphragm can be source of a leak.

In general plugging off one vacuum circuit at a time allows you to find what is leaking because when it is plugged off full vacuum is restored.