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Ok, very weird situation: 2001 Mitsubishi Montero Loud hissing sound coming out of front passenger wheel well when accelerating.

Here is a video: https://youtu.be/tiSFdoniRm0

I have put it in park and started the engine, gassed it until 3500 rpms, no hissing sound at all when parked/not in gear. And I have done a visual check of all vacuum hoses, and they seem fine.

If it only makes a loud hissing sound when its in gear, does it mean its the transmission?

The hissing sounds like pressurized air escaping, it pulses with the RPMs, its not a continuous spray of air, and you can hear it from around 2250 rpms to around 4000 rpms, at anything lower than that you can't hear the sound. I also have a lot of power loss when going up hill or trying to accelerate quickly.

If it was a vacuum leak, or an exhaust leak, you'd be able to hear it when parked and revving at high rpms, right?

It only happens in gear, so it seems like a transmission issue.

I checked the trans fluid and it was pretty low, so I put a quart in and the level is between the two notches on the dipstick.

What could be causing this sound?