2011 Ford Escape, 3.0L V6; 112,000 miles

I need some help identifying a new vibration in the engine. I just completed a tune-up with following items and now have a slight, but noticeable engine vibration at idle, with or without the AC on. It's not noticeable at any driving speed.

My tune-up consisted of:
- spark plug replacement (platinum plugs)
- intake manifold gasket replacement (recommended, since I took the manifold off to get to the rear plugs)
- serpentine belt
- water pump belt
- air filter change
- oil and oil filter change
- (I didn't replace the PCV valve, but will do that soon)

Since the vibration started right after the tuneup, I don't think it could be motor mounts - they feel flexible and are not leaking any fluid. My next step is to lift out the fuel sender/pump unit and check the inline filter at the bottom (Apparently there is no fuel filter on this model!)

Any ideas on what to check for? Bad "new" spark plug? Intake manifold not on securely?

Thanks!