Hey i've a similar post to this before but I wish to discuss this progressing problem as to what might be the cause of it? Is it serious or typical?

When i start my car (cold) I can hear the sound of water swishing by comming from the middle to passenger side of the dash. AND -- when I press the gas and rev the engine, it makes the noise... but after it warms up, the sound is gone.

Any thoughts, experiences or opinions?