I keep noticing this, thought it was maybe a fluke, but realized that it happens all of the time. Just off throttle, my 03 GT seems to have a pretty rad intake burble. Fairly certain it's intake at least, sounds like it to me.
I'm wondering if it's the removal of the elephant snout intake snorkel behind the driver's headlight. I damaged mine during removal and haven't moved forward with replacing it yet.
The GT is stock, though it does have a TRD filter installed by the previous owner.
Just checking in with the hive to see if this is a commonality, as it sounds cool as hell.