Mandingo

I think what you are describing it the perfectly normal 'hiss' that our vq makes.

I only hear it on deceleration at about 3k rpm though. I've been asked if my car is turbocharged at a stop light haha. I guess it almost resembles a long, drawn out blow off valve noise

Edit:

If it doesn't do it while revving, then it might be something making light contact producing a high frequency vibration noise like you said. Get under the car and check every possible area around the cats for something touching or nearly touching.

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