Coolant draining noise when accelerating

Hi, wondering if anyone has any ideas/suggestions about this. This is my second 2004 Grand Prix. I had a GT2, now have a GTP Comp G. Whenever I accelerate (warm or cold), I can hear the coolant draining from the overflow resevoir. It's pretty loud and is starting to get under my skin. I never heard the sound before in the other car, but noticed it immediately in this one. Any ideas how to stop or make it less noticeable? I recently had the rad flushed, maybe there wasn't enough fluid put back in?

How much fluid should I have in the resevoir when it's cold? 50%, 75%? I'm open to ideas.

Thanks!
Oh, I couldn't find a similar thread, so I apologize if I didn't find it using my best search terms.

One possibility is air in the cooling system so try a bleeding any air out. The reservoir should be filled to the hot and/or cold marks. But with engine cool make sure the level at the radiator is good regardless of what is in the reservoir.

Another possible cause of the gurgling sound you hear through the heater core is a partially stuck open or improperly operating thermostat.

I have recently installed a 180 degree thermostat from ZZP, the problem was there before this was done, the o-ring for the thermostat was snug and in great shape. Just to clarify, if the engine is cold and the resevoir is lets say half full, will it not leak out when I open the rad cap to top it off? It just started snowing here, so avoiding any potential problems would be great. =)

I had a similar problem. It turned out my radiator was partially plugged as the previous owner had used stopleak. Tried flushing with less than desirable results. Purchased a good used radiator from a scrapyard and haven’t had a problem since.