2006 Mustang GT auto trans hiss

I just recently put a quieter exhaust on my 2006 Mustang and now I’m hearing a bit of a hiss from the transmission when it is in Drive or Reverse. I’m neutral or park, it is totally quiet.

I’ve read in some older posts doing a search on google that others talked about this and some said it was normal and others said they have no noise. It’s especially noticeable in a drive through or in my garage (with the windows down) where the sound probably has something to bounce off of.

Any ideas, or is this normal?

You sure it's the trans? I had a hiss/slight whine that seemed to come from the trans for a while. Then my alternator took a dump. Replaced it and the noise was gone.

It sounds like it is coming from under the car and I only hear it in drive or reverse. Definitely seems to kick in when the transmission is engaged.

Have the same, its normal. 33.000 miles on it.

RedGT

Hissing like a snake?

Mine doesn't hiss, 5R55S trans...

Hiss is usually when a catalytic converter is clogged, and exhuats is leaking
before the cat. Believe it or not, the engine is under a 14-17% load when
in stopped in gear, idling; at least that's what the OBDII data says. Does
the hiss increase in volume with fuel pedal?

These transmissions do have a vent tube that connects to the bell housing, so
that would be the only place I can see it coming come from, if it is the transmission.