Cooling system burping on 4000 or I5's

[DK]

I drained the block and the radiator. I left the system open for
couple of days, as I had the wrong waterpump on hand and didnt want to
put it back together with the leaky pump. It got a bit rusty, so I
filled it up with water for now, and will flush it thoroughly before I
put some real coolant into it.I did fill up the block trough the upper
radiator hose ( blew a Honda HG by not doing so, bdtd).