I don't think it is just ecotec ignition coil cover as I feel the vibrations in the cabin. You are right that the cover sometimes makes vibration sound but I don't think this is it.
I am afraid of tightening the valve cover bolts as they are to be at 71 inch pounds but mine are around 78 inch pounds and I am afraid they might get stripped. Maybe I should put in new valve cover gasket as it has some mild oil leaks.
yeah don't tighten down the valve cover bolts further, the only solution for the oil leak would be to replace the gasket. It's not too hard of a job and the gasket is relatively inexpensive.
As for the vibrations, you could remove the coil cover to test it out, but if it's vibrating the cabin more than likely it's your mounts. I had the same issue for a while and replaced the passenger side engine mount and both transmission mounts and it made a world of a difference sound and comfortability wise, it felt like a new car for a little bit lol. It was about a 45 min job with a socket set and car jack. I can give you part numbers for the engine/transmission mounts and can dig up the diy video I used for reference.
on the valve cover gasket leak note, I would check out how much oil has leaked into the spark plug valleys, as when the rubber gasket becomes hard and brittle oil can leak into there.
