What's the difference between P and D?

[PlanZero]

This is probably a stupid question, but I am really trying to learn the complexities of the automatic transmission.
I've been severely neglecting my 97 Tsi awd, and since I "sold" her to my GF I've only been doing the necessary maintenance on her. She always felt sluggish boosting uphill. She also has always sounded like a jetliner taking off, which I realized later on is way too loud for only having an injen intake and evo bpv.
Anyway, I've come to realize she has what sounds like a massive intake leak somewhere around the manifold, but only in Drive or reverse, never in park.
When I put her in drive with the ebrake on I can hear this whooshing/hissing sound and when I squeeze the stock UICP it changes pitch. I will be doing a thorough BLT shortly, concentrating on the manifold gasket, injector o rings, egr gasket, vacuum lines, etc.
But in general, on an a/t what is the main difference between Park and the other gears? I realize it's under load when disengaging from P, but why does the intake tract all of a sudden start sucking so much more just shifting gears? Thanks in advance.

 

[Dstarperformance01]

When the trans is in park it's spinning the pump freely and not applying hydraulic pressure to the forward drum and clutches. How much rpm change from park to drive or reverse? It should only drop a couple hundred rpm. What is the condition of the throttle body? Lots of carbon on the edges?

 

[PlanZero]

About 1100 in P and 800 in D, and idles perfect. I finally got around to putting on a rebuilt TB and deleting cruise control yesterday and noticed a big improvement; I knew the shaft seals were shot awhile ago. But is still here this whooshing sound in gear, even at idle. I plan on doing another BLT Monday directly on the TB elbow.

 

[Dstarperformance01]

Okay. Posting a video would help. 1100 rpm idle in park is way to high. Idle should be around 750-800 rpm.

 

[PlanZero]

Actually, 750-800 in park is too low. 900-1100 in park is optimal for an a/t DSM, since it will drop to 750-800 in drive (optimal idle). If it was only 800 in P it would drop to 500 in D, too low. That's factory specs (stock cams) as I had done a ton of research a few years back when tackling an idle problem on my first a/t DSM.
And in my own experience, that's where they perform best.
No need for a video, as you wouldn't even hear the hissing over the engine noise. It's definitely a leak in the intake tract, I'm only asking why it's only prevalent when put into gear. Thanks