I don't know how to describe it, but I was told by the guy who works on it that it's a "signal" to turn off the numerous transmitters off. If you have the PL programmed on the output you don't hear it.
What I've heard, off the Plainsboro tower, is only just before the repeater de-keys, not after the end of each mobile/portable transmission. Often I can hear the last mobile's signal drop out well before (0.5-1s) that noise.
Also, I thought I've heard that noise coming through my PL squelch. That's why I usually lock out that channel. This is on radios which are usually pretty responsive to changing PLs in the middle of reception.