I have.

It can be difficult to describe what something sounds like to someone else - a recording might help ID the cause.

It didn't happen today when the car was parked facing downhill. Could also have been because it was warmer this morning... sigh

Check your carpeting under the dash to see if it's damp. If it's damp. Your A/C drain under the hood could be blocked. Seen it happen with vehicles that have been undercoated.

It is not damp.

Ok, the carpet is dry. So we can rule that out. When that drain gets block. The condensation from the A/C has no place to go to. And it fills up the area around the evaporator. Then it leaks on the carpet in some vehicles. And it will sound like what you described.

This sound like a significant amount of liquid, it really bugs me that I don't know what it is....

You need to get someone to ride along and try to locate the source of the noise while you concentrate on driving. The tube from a roll of paper towel with one end placed by the ear will help zero in. Going by your descriptions, it may take a few tries.

It may be worth getting someone to bleed the air from the engine cooling system just to eliminate that possibility.

I haven't been able to reproduce it again when it's been parked since my initial post, but I have been able to reproduce it while driving. When I'm stopped at a light it makes the sound as I accelerate from the light. The one consistent variable is the air conditioning, it has always been set to auto. I'm going to drive it all weekend without the a.c. running top see if it stops...

Do you have a cai?

What is a cai?

IIRC, the A/C runs to dehumidify the cabin even when you turn it off at the control panel. You may have to pull a fuse to turn it off completely.

Edit: CAI = Cold Air Intake

No, I don't have a cold air intake.