Crown Vic weird noise

2000 Crown vic, 93,000 miles, as the temperature has risen this spring (into the 80s now) I've noticed a new sound when I drive. It begins only after spending about 10 minutes going 70+ on the interstate. It is very faint and only occurs when I first accelerate (in other words, if I'm going 40, and accelerate, I will only hear the noise till I hit 50, then it seems to go away as I continue up to 70). It is sort of a gurgling sound, like maybe something isn't getting air/fluid correctly? I'm takingf a wild guess it may be some kind of problem with my exhaust that occurs only when the internal temperature increases (which is why it would only happen after I've been driving a while). Weird thing is I only hear this going HOME from work when it is hot outside, not when I go TO work (when it is cooler outside).

I wish I could tell you where exactly the noise comes from, but it just sounds like it is coming fdorm in front of me or below me. If I put the car in park, and roll the window down, and step on the gas, I cannot duplicate the noise.

Any ideas?