You hear Riverhead just fine, because it is simulcasted through the entire county. I'm not sure why the 1st and only the 1st sounds choppy to you, but it might have something to do with the trunk receivers (voting system), or the pd patch system.
3536 should be the Town of Brookhaven services. I believe it was brought up on the RR Forums a few times. It is listed in the RRDB under Misc. services: Suffolk County Trunking System, Suffolk County, New York - Scanner Frequencies
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