Except for maintenance, you probably should never drain the system! If you hear gurgling, and it is a hot water system (i.e., not steam), then you have air. To get the air out, the last thing you want to do is drain it. Somewhere, there needs to be a fill valve. Also, you need to know what the system pressure is so you need a pressure gauge. Depending on how many stories the water has to be pumped, would somewhat depend on the normal working pressure, but many are around 15psi. When properly set up, there is NO air in the system, and it is pressurized to the system design. A pump often cannot self-prime and push water through an air pocket, so you need to go to the highest point, find a place where you can purge air while filling it. WIthout seeing your system, it's hard to tell how it was designed to be done, but somewhere there is likely a connection to the cold water supply, connected through a backfill preventer, into the boiler water system. Sometimes, there's an autofill valve, but it may be shut off and unable to maintain pressure in the system. Also, you will have some sort of expansion tank in the system, and it may be waterlogged. Depending on the type, it may need to be drained, or if it is a bladder type, replaced. This is required since when you heat water and the system is full, the water expands...it needs to go somewhere, and that is the expansion tank. WIthout it, the pressure will rise, the safety overpressure valve will (or at least should) open, to drain off enough water to maintain the max pressure. But, when it cools off, the water shrinks, and might suck air into the system, leaving you deficient.