Hum could be from the circulator...flowing water doesn't usually do that unless it goes through a valve with something loose that can resonate.
You don't have to have a gauge on the system, but it sure does help verifying it is working correctly, so, for a short-term solution, yes, you can buy a plug and plug the hole where it is attached (or maybe a cap, depending on whether it is male or female). But, doing that will mean draining off enough to remove any pressure, and then refilling the system, and without a gauge, it's really hard to do and get it correct! You could screw one onto a drain valve - you can buy pressure gauges that will screw right onto them or any hose bib.