Engine Rumbling Knocking Noise 330xi

Just changed out plugs yesterday. (2005 330xi) Car really ran smoothly and quiet. Short trip today.On the way home I noticed a knocking sound. Kind of deep underneath the normal engine sound. Not sure if this is related to a simple plug change. Also changed air filter, cabin filter, spring day off activities. Is this sound something anyone can pinpoint or has experienced before. No codes or CEL. No apparent leaks or blown hoses or what not.
Here is a link to a short video. Underneath normal engine is a rhythmic thump.
Any Clues? Or ideas where to start are appreciated.

I wish my car was idling as well as that. Sorry, I'm no help.

I'm sure you put that silver metallic cream on the spark threads before you put the spark plugs in. I'm sure you're more experienced then I and wouldn't overlook such a thing.

When I did my spark plugs for the first time I didn't know I needed that silver cream on the threads. The auto parts person was the one who educated me on it and saved me from myself. He told me I'd have all sorts of problems if I didn't do it.