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.
Jim