How and where is your coax routed? Have you moved the scanner to different areas of the room? Does this hiss happen on all bands ie police, city services, airplane bands?
Might try grounding the scanner chassis to a water pipe to see if hiss increase/decreases/no effect. I suspect there is and intermod or birdie as we hams call it that is creating a spurious frequency that falls in the 160MHz passband.
