Nothing modified - the blades are the same. Boeing were trying to progress the composite ACRB (Advanced Chinook Rotor Blade) as part of the Block II CH-47F upgrade, and that may have different noise characteristics as they featured swept tips (to smooth airflow separation - they are similar in concept to the blades proposed for the RAH-66 IIRC). However, despite showing promising improvements in lift, it vibrated badly and has, last I heard, been shelved for now. Many moons ago at Odiham we were instructed to slow down to less than 120kts inside the MATZ to be 'good neighbours' as it reduced noise (albeit at the 'cost' of us being airborne longer…). AT 90kts, it's broadly min power speed so neither engine nor rotor/transmission system are under high loading, and at low level the footprint is appreciably smaller. Careful planning and the use of wind and topography can yield some very interesting results, and you can get surprisingly close sometimes (especially at 50ft) before you're heard - and as MGD noted earlier, reflected noise can also help to confuse an opponent.