Gurgling, swishing, liquid noise behind dash noticed after rapid engine state changes - 2008 Prius

I've got a gurgling, swishing noise issue on my '08 Prius. I hadn't heard or noticed this noise prior to last weekend, over which I had my water pump (two weep hole), PCV valve, belt, and 3-way coolant flow control valve replaced (engine code p1121). The mechanic replaced the drained coolant with new SLL pink coolant I bought from the Toyota dealer. My Prius has 186,000 miles on it and the p1121 code had probably been active for about 18 months preceding the replacement. The check engine light is now gone and has yet to return, but I am now hearing a swishing noise from behind the dash area at times. It is most audible during the 1st drive of the day with the A/C on. As the vehicle runs through its warm up cycles, I'm puttering to the end of my subdivision. This is the first opportunity for my car to come to a complete stop. When I do, and I push the pedal hard down, the combustion engine basically comes to a stop; and it is at this time that the gurgling and swishing noise is emitted. After this, it is possible to be heard upon subsequent accelerations following a complete stop, but much less audibly. I do have some extra clean coolant, so I will try taking steps to remove any potential air caught in the system, but I'm not sure if there might be some other cause. Anyone got a different idea as to the cause?

Possibly related, and prior to the repair work mentioned above, I recently started hearing a sort of ghost moaning-groaning noise when I would turn on and turn off my car. I thought it might be related to the failure for coolant to be pushed/pulled to the thermos (i.e. related to p1121), and this noise is what prompted me to get the p1121 code finally addressed. However, the sound remains. Anybody know what might be causing this?

 

Yeah, I guess I figured on that. After taking the DIY path and filling, running, refilling, repeating, it seems to be OK now, though I haven't completely filled the reservoir at this point. Is that the expected status? To have a 100% full reservoir + a seemingly full radiator?