Trans slip or something else?

So I was driving my 998 last night around 35 and I went to stop but quickly changed my mind and went back to the gas. When I did that I heard a weird chshhh for a second. For a second I thought a truck with airbrakes was nearby.
Was this the trans slipping, or some weird injector system noise?


Thanks,
Mike

It was probably the Cadillac valve. It makes a kind of a hiss.


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I should also mention it did it at least 1 other time that trip. On the brake for 1 second then right back to the gas and chshhhh. Sounded like it was under and not in front... But with all the noise these trucks make it's hard to be certain lol.


Mike