Air or vapour in the cooling circuit, probably caused by low coolant level
or headgasket failure, undo the coolant filler cap after a running the
engine for a little while (be carefull, there might be pressure in there)
smell inside the reservoir, If it smells exhausty, or petrolly, then the
headgasket has breached, otherwise check the inside of the oil filler cap,
the presence of quantities of white gunk indicate possible head gasket
breach, otherwise you have a normal coolant leak somewhere, the rad cap
isn't sealing properly, or it just hasn't been topped up or checked for
ages.

Top up the coolant now!!! If the coolant is low enough for vapour/air to be
being pumped through the heater matrix, then the level is critically low,
and you are risking overheating your engine.

Ken Phillips

I used to swap the hoses around after flushing the system on my Fords
.Reverse flushing sure helps sounds like the gunk is  slowing down the flow
of liquid .