Another mysterious thing with cooling system

[geradot]

I have a very strange thing with my Legacy-2.2 L, 1991 (170k) after water pump replacement few months ago: at well heated engine while rpm are dropped from 3000 to about 1300-1700 I can hear dull, gurgle-like sounds from somewhere under the hood. I think the source is in cooling system because that sound could be heard more distinctly when heater is fully engaged. With all this going on the engine temperature in normal (slightly below of mid-scale), heater is operating normally, engine runs smoothly, coolant level is normal, no coolant leakage, no exhaust smell from coolant reservoir, ignition is correct, timing belt is almost new, belt tensioner is good, no coolant signs in oil, water pump and thermostat are operating normally, radiator cap is checked.
The most funniest thing is that it is not possible to simulate such a situation and sounds at stop position but it regularly appears while driving and engine is well warmed-up.
That knocks sometimes irritate but mainly arose anxiety and desire to stop and to look under the hood (still without any success).

What could be the possible reason for that all?

Could such a sound be produced somewhere out of cooling system (for example backfire in intake or in exhaust duct)?

Thanks in advance for any comment

[Legacy777]

When you replaced the water pump.....did you remove the bleed screw on the passenger side of the radiator while filling/bleeding the air out?

[geradot]

When you replaced the water pump.....did you remove the bleed screw on the passenger side of the radiator while filling/bleeding the air out?

Oh Yes. I read Haynes manual on Subaru very carefyully. So not only that top bleeder was open but the heater core was fully open too (as he recommends) and antifreeze was poured very slowly. Nevertheless...

Some guys here conclude that "cough" is coming from exshaust collector due to rich mixture which in its turn could be caused with sticked valves, faulty knock sensor, leaking injector, air duct ...
If yes, coud this happen without CE error?

[geradot]

Enother suggestion:

found it at http://www.2carpros.com/topics/cooling.htm
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Question: 1999 Subaru Legacy mileage: 70,000. My car makes annoying water bubbling/flowing noises under the dash when throttling up or down (especially after idle). I suspect that it needs to be bled correctly - however I've tried removing the bleed screw, and then filling, with no success... Is there something I've missed or some trick to this model? I've taken the car to 4 Mechanics (1 was a Subaru agent) - all have failed.

Answer: It sounds like your water pump may be pulling air in at higher rpm. A new water pump should solve your problem.
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How could it be? Water pump pulls air in? without coolant leak?