It does sound like the engine is "winding down" or something to that effect:

Here goes my best approximation of what occurs:

Let out the clutch, give it gas and I get the good B20 pushrod engine acceleration sound. Let off the gas and push in the clutch at a higher RPM and when the clutch is in I hear the "winding down" sound I tried to describe earlier coming from the engine (I think). It is happening in 1-2, 2-3 and 3-4.

I've had a noisy thrust bearing in another vehicle and it doesn't sound like that...this latest go at it has me thinking that it is the engine making the sound - though it ~could~ be a pilot bearing since I've never heard one before.

Any thoughts? Am I making something out of nothing? It got me curious because I hadn't heard this sound before.

It doesn't make any noise when the clutch is engaged that would make me think it is the thrust bearing.