Golf IV / Jetta IV :: Buzzing / Hissing Noise At Certain Speeds / RPMs / Loads

I have a 2002 Jetta GLS 2.0 4-speed automatic, 151,xxx miles... I have only owned it a couple of thousand miles, so I can't offer much on what is "normal"...

The issue I am having is as follows: I sometimes get a strange buzzing/hissing noise that sounds like it is coming from the engine compartment ~45 degrees to my right while driving. It's very hard to describe - the best I can say is it sounds a bit like a little plastic gear is spinning/grinding at high rpms, yielding a buzzing noise, and there is a hissing-like sound as a backdrop.

The sound happens only occasionally (but seems to be happening much more frequently, which is worrisome). I have heard the sound in different driving scenarios, so I can't say "it only happens when...". However, it DOES seem to be sensitive to throttle and driving conditions in some way. I have heard it while a shift is taking place, I have heard it under load on a long incline, and I tend to hear it right around 60mph. Interestingly, in the 60mph example, can make the sound go away by accelerating past 65, or by decelerating down to ~50... it seems like there is some kind of transition taking place at 60 that fosters the sound?

The transmission is generally functional - I had the fluid changed right after buying it (with the correct fluid), and it generally shifts ok. It does have some of the quirky behavior the transmission is known for: occasionally hesitates to go from R to D, sometimes shifts while driving are less subtle than they should be. But I'd say as a whole it operates as a 4-speed transmission would be expected to.

However, since this strange noise is getting worse, and seems to be related to shifts/load/andor speed in some way... I am worried about trans health. What to check? Because it is intermittent, and doesn't exist at idle and rarely appears when going around the block good luck getting a mechanic to observe it...