My baseboard heat seems to make a lot of "rushing water" sounds that seem to be getting louder and more frequent... any ideas?

One word "air". You have air in your system which makes the system sound like a babbling brook. This is usually caused by low pressure or improper air elimination.

This is an old post but--what should the questioner do about it?

Purge the system, blead any air vents in the system on the baseboards and verify the water pressure on the boiler gage is at 12 - 18 psi if the home is not more than two stories.