The sound of reel-to-reel tape coming to a stop while in play mode. There are tape simulation plugs that have that effect, but you could also mess with creating your own effect as you described above, and/or try modulating a chorus or delay program (an analog unit would be great). Or simply sample this effect (or the similar-but-different "turntable back-spin") and play with insert editing-in the sample where you want it.
GJ