Strange suction sound when accelerating.

Ok, here's a strange one. Every once in a while when accelerating full throttle, I get a strange suction noise that sounds like liquid being sucked up but with air in the fluid. Sort of like a straw when a drink is empty but on a larger scale. When I release the accelerator, it stops and returns when I get back on the gas. It usually only lasts for a minute or two, then goes away. It sounds like it's coming from the firewall area just in front of the driver. The only thing in that area that has fluid is the power steering reservoir, but the level is good and no leaks. Any suggestions of what to check?

 

Performance is good and idle is smooth. My first thought also was maybe HVAC related, the coolant lines to the heater core maybe, but I think those are on the other side of the car. I did just change all my fluids as I do every winter (oil, gearbox, coolant), so maybe coolant, but the level seems good and no air/steam comes out when I crack the bolt on top of the elbow at the front of the engine, so I think the coolant is OK, but worth checking again for sure. Whenever this has happened, I've tried turning off the climate control and adjusting the temp to full cold assuming that would stop any flow to the heater core, but maybe it doesn't, and doing this did not stop the sound. Detroit steel, which HVAC line was plugged on your car? It's worth checking out.

 

Yup, it may not be that at all, but easy to check out and as you said, costs nothing. Thanks.