OK , probably not engine brake related , particularly if you can't hear a real difference in engine tone and probably wouldn't get to 1,500 RPM.
I had to ask about exhaust brake as the ISC is one Cummins engine model available with, either exhaust or engine brake.
BTW: Jacobs , manufactures both exhaust brake and engine ( compression ) brake systems . So even the term " jake brake " can be confusing.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.