What type of radiator/baseboard is this?
OK I got it.  I had one of those in my kitchen, under a counter.  Not sure of the official name for it, but I've heard it called a "heatilator".  It's basically part of your baseboard hot water heating system.  I think the purpose of the metal cover is to provide a sort of chimney effect, where the rising air gains a bit of velocity and helps pull cooler air from the floor through the fins.

Here's picture of mine from when I was tiling the floor.



Sounds like you might have some air in the lines.  There should be provisions to bleed them - a little valve of some sort near the highest point in the run.  Could also be multiple bleed valves.

Bleed valves vary in shape and size.  Sometimes they look like a garden hose spigot, and other times they're more like an automobile tire Shrader valve.