Faint antifreeze smell inside car after extended idling

2013 Prius 3

Last Sunday, after idling for about an hour with the heater going, I smelled faint odor of antifreeze. The same thing happened a couple of days prior after idling for about an hour. I haven't smelled the antifreeze while driving (extremely limited since this issued showed up). The car has about 75K miles on it.

The reservoir is full to the line and the cap *seems* tight. i have no idea if the car is overheating because no temp gauge(?). I didn't see a puddle under the car in the driveway, but that's not where I was idling.

The car has also been making a funny metallic noise, maybe vibration, ever since I had the oil changed a few weeks ago. Mostly subzero temps and the usual snow, ice, and clods.

What is likely wrong? What should I check first?

Thank you.